TODO

Near-field to far-field transformations.

Gyromagnetic materials (via general anisotropic conductivities).

Finish implementation of active materials (multilevel atoms and
saturable absorption/gain).

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

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

Use more-stable algorithm for dispersive media.

Field dump/load (i.e. restarting)?

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

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

Integrated support for calling MPB mode solver for current distributions.

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

Dump field Fourier transforms in a readable, documented format.

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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LIBCTL FRONT-END:

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

Support arbitrary-shaped sources.