API Refactoring to export symbols in submodules explicitly

[youwuyou]

As addressed in issue #44 , it would be desirable to refactor the current API and ship it with the next release of Chmy such that users can simply use using Chmy instead of concrete import of submodules as we have now. This PR starts with a state where all submodules have been refactored besides two, which we will adress below.

With the aim to avoid the usage of additional Reexport.jl package, we followed a similar approach as used in Oceananigans.jl.

The remaining things to do include the followings, I would like to discuss with @utkinis and @luraess and see how we want to best do it before further refactoring:

• Explicitly export Grids and GridOperators, ➡️ accordingly remove usage of using Grids and using GridOperators in the test suite
• get_backend() of Architectures module name clashes with KernelAbstractions.jl ➡️ Thus is test suite the module is used explicitly yet

[youwuyou]

To the first item: the Grids and GridOperators submodules contain parts where the symbols are exported in a rather exotics way (ref. cartesian_field_operators.jl for an example), how do we want to reexport these expressions?

[youwuyou]

To the second item, currently users need to explicitly use KernelAbstractions.jl (and I think it is good this way). But possible name clashes could be problematic, do we want to rename the relevant functions or is there maybe a workaround for it too?

[youwuyou]

Some clean-up jobs would be to modify relevant parts of the documentation & the examples (this is not necessary, because using submodules works too even with our API change, but maybe good for users to avoid confusions?).

[utkinis]

To the first item: the Grids and GridOperators submodules contain parts where the symbols are exported in a rather exotics way (ref. cartesian_field_operators.jl for an example), how do we want to reexport these expressions?

I suggest doing it in the most straightforward way. These "exotic" exports are only needed to generate functions for Cartesian grids with coordinate names x, y, z, which will probably change in the future, but not from API perspective. So exports from Chmy.Grids and Chmy.GridOperators will contain following lines:

# Grids
export coord, xcoord, ycoord, zcoord
export Δ, Δx, Δy, Δz
# other exports...

# GridOperators
export δ, δx, δy, δz
export ∂, ∂x, ∂y, ∂z
# other exports...

[utkinis]

To the second item, currently users need to explicitly use KernelAbstractions.jl (and I think it is good this way). But possible name clashes could be problematic, do we want to rename the relevant functions or is there maybe a workaround for it too?

Yeah, I should've done this long time ago, here it should be:

KernelAbstractions.get_backend(arch::SingleDeviceArchitecture) = arch.backend

which would remove the name clash

[luraess]

Are we good to go with this one @utkinis ?

[utkinis]

Thanks for the work @youwuyou and @luraess ! Looks good to me, I guess it deserves a non-breaking release