TODO

Skewed bloch periodic boundary conditions for non-orthogonal unit cells

Discrete rotational symmetry in cylindrical coordinates.

Second-order accurate subpixel smoothing scheme for
dispersive materials with complex permittivity.

Support 3d (r,phi,z) cell, where m is "Bloch wavenumber"
      -- gives 6-fold (and n-fold) symmetry as side effect
      -- allows computation of bent photonic-crystal waveguides

Implement transparent boundary conditions to back PML layers, so that
glancing-angle waves can be absorbed.

Re-entrant functions: foo(vec&) arguments should be foo(vec&, void*)
where the second argument can be used to pass state (instead of global
vars).

Use proper PML for cylindrical coordinates, rather than quasi-PML.

Use more-stable algorithm for dispersive media.

Make sure epsilon and other material properties respect symmetry,
periodic boundaries, etc.?

Remove monitor point class (redundant with HDF5 output, DFT volumes?).

Rename fields::initialize to fields::add_to_fields or something like that,
and allow specifying a geometric_volume.

Support Pade approximates (libpadespectrum) for Fourier and modal analysis.

Check sensitivity to rounding error and if there is a better way to handle:
      structure.cpp:555 (PML region boundaries)
      vec.cpp:560 (interpolation weights)
      anisotropic_averaging: 79 (magnitude of normal vector)

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Python & Scheme Interface

Need a way to reset epsilon when restarting, and also phasing support.

Support arbitrary-shaped sources.