Transformation general doc

docs/CMakeLists.txt

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         "${CMAKE_CURRENT_SOURCE_DIR}/going_further.md"
         "${CMAKE_CURRENT_SOURCE_DIR}/uniform_heat_equation.md"
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docs/transformations.md

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+# Transformations {#transformations}
+<!--
+Copyright (C) The DDC development team, see COPYRIGHT.md file
+
+SPDX-License-Identifier: MIT
+-->
+
+On the top of DDC core - which provides the basic tools for the needs of performance-portable physical simulations
+(describing meshes with *DiscreteDomain* and representing physical fields with *Chunk*) - are build computation kernels
+to perform transformations. At the moment, the two available transformations are Discrete Fourier Transform and Spline methods.
+
+Those methods are performance portable (supported with CPU Serial, CPU OpenMP, CUDA and HIP backends). They are used
+in *examples/heat_equation_spectral.cpp* (for DFT) and *examples/characteristics_advection.cpp* (for splines).
+A particular use-case of splines is benchmarked in *benchmarks/splines.cpp*.
+
+## General presentation of transformations
+
+### Computation kernels dedicated to transformations in DDC
+
+A transformation is a change of basis, going from a function space to another. Because the available memory is finite,
+the bases are necessarily finite (which means it contains a finite set of basis functions, but those basis functions
+may be continuous).
+
+There are many use-cases of transformations for simulation or signal processing needs: interpolation, spectral methods,
+finite element methods, filtering (compression), signal analysis (post-process)...
+
+Every basis has its own specificities (ie. orthogonality, n-derivability, etc...) but as they all are
+function space bases, they have in common a formalism and a terminology. However, this is currently
+not very manifest when looking at the API in the DDC implementations (DFT and Splines API are very different).
+It must be explained:
+
+- Fourier requires the periodicity of the represented function, thus boundary conditions do not need to be provided.
+- Fourier basis functions are indexed with the wave vector \f$ \vec{k} \f$, whose possible values form a set of coordinates which generate
+the Fourier space. There is a bijection between the mesh of the "real" space and the mesh of the Fourier space.
+The situation is more complicated for Splines.
+- Fourier and Splines basis use-cases are quite different. Thus - as shown below - the so-called "evaluator" is not
+implemented for Fourier (while inverse DFT is) and inverse Spline transform is not implemented (while Spline evaluator is).
+- FFT backends support 1D to 3D Fourier transforms. General n-dimensional Spline transform would be a mess to implement for all supported boundary conditions so we limit to the 2D case.
+- DFT is not currently supported in batched configuration (but it may be the subject of future development). Batched splines are supported.
+
+To summarize the available features of the two kernels:
+
+|          | Direct transformation | Inverse transformation | Evaluation |
+|----------|-----------------|--------|-------------------|
+| DFT      | *fft*           | *ifft* | x                 |
+| Spline   | *SplineBuilder* | x      | *SplineEvaluator* |
+
+So, even if the two kernels *could* share a common API, this is not currently the case but this is more due to pratical considerations than intrinsic mathematical differences between the two methods.
+
+### General transformations theory
+
+The simplest way to describe a discrete function \f$ f \f$ is to provide its value \f$ f(x_i) \f$ at every point of the mesh which supports it.
+
+Another more general method is to define a basis \f$ \phi_j \f$ in a function space and provide the set of coefficients
+\f$ c_j \f$ which generate the discrete function. The set of coefficients is thus another way to represent the discrete
+function:
+
+\f[
+f(x_i) = \sum_j c_j \phi_j(x_i)
+\f]
+
+Note: the first method is covered by the second one in the particular case of \f$ \phi \f$ being the Dirac basis functions.
+
+The aim of a computation kernel dedicated to a transformation is to determine the values of coefficients \f$ c_j \f$
+knowing \f$ f(x_i) \f$ (and for the inverse transformation, \f$ f(x_i) \f$ knowing \f$ c_j \f$). 
+
+Additionally to the *transformation* and the *inverse transformation*, there is a third operation which can be useful:
+the *evaluation*. Indeed, more than a discrete function, coefficients \f$ c_j \f$ generate an interpolating (continuous)
+function which can be evaluated anywhere between the points of the mesh.
+
+Note: performing a (direct) transformation then an evaluation is an interpolation.
+
+Bases of function spaces can have additional properties like orthogonality
+\f$ <\phi_i|\phi_j>= \int \phi_i(x) \phi_j(x)\; dx = \delta_{ij} \f$ (thus, \f$ c_j = <f|\phi_j> \f$).
+This is the case for Fourier transform, and it provides a numerical method to perform the transformation (but not the most
+efficient one). In the general case orthogonality is not verified though, thus transformation algorithms are basis-specific.
+
+### Lexical 
+
+Here is a summary of different general terms used in the context of DDC transformations:
+
+| Term     | Meaning & commentary |
+|----------|-----------------|
+| Discrete function | Function defined on a mesh (via its values at each point of via its coefficients in a particular finite basis representation). |
+| Dimension(s) of interest | Dimension(s) along which the transformation is performed |
+| Batch dimension(s) | Dimension(s) unaffected by the transformation. It leads to a set of transformations performed independantly (along dimensions of interest). Those are embarassingly parallelizable. |
+| Interpolating function | Continuous function generated from the coefficients \f$ c_j \f$ and the basis functions \f$ \phi_j \f$. |
+| Approximation | Same as transformation (computing the coefficients of the interpolating function). |
+| Interpolation points | Points of the mesh, supporting the discrete function through which interpolating function goes. |
+| Evaluation points | Points on which we want the interpolating function to be evaluated. |
+
+## Discrete Fourier Transform
+
+Fourier transform is the transformation with \f$ \phi_j=e^{ikx} \f$. The algorithm used is the Fast Fourier Transform.
+
+Fourier space associated to continuous dimension *X* is represented using the *Fourier<X>* templated tag.
+
+The performance-compatibility is ensured using the backends FFTW, cuFFT or hipFFT depending of *Kokkos::ExecutionSpace*
+which templates the functions. Thus, no FFT algorithm is actually implemented in DDC, which is more a wrapper of those
+backends. However, the spectral mesh can be computed (as a *DiscreteDomain*) knowing the real mesh (which is not a
+commonly-implemented feature in the well-known FFT libraries).
+
+## Spline transform
+
+Please refer to Emily Bourne's thesis (https://www.theses.fr/2022AIXM0412.pdf) and the spline-related API documentation.