High level get_eigenmode_coefficients

[ChristopherHogan]

This will require a documentation change, but I think Ardavan is working on that documentation now, so it will need his PR first.
@stevengj @oskooi @HomerReid

[ChristopherHogan]

When called from Python, get_eigenmode_coefficients is unable to infer the direction, resulting in

meep: cannot determine normal direction in get_eigenmode_coefficients

Looking at @HomerReid 's wvg-taper.cpp (which has been deleted from the repository, so I had to look at an old commit), he calls the overload of add_dft_flux that takes a direction as the first argument. Then, get_eigenmode_coefficients gets the direction via dft_flux::normal_direction.

However, in Python and Scheme, the overload of add_dft_flux that takes a volume_list as the first parameter is called, which returns a dft_flux with NO_DIRECTION as the direction. This happens in _add_fluxish_stuff.

def _add_fluxish_stuff(self, add_dft_stuff, fcen, df, nfreq, stufflist):
        vol_list = None

        for s in stufflist:
            v = Volume(center=s.center, size=s.size, dims=self.dimensions,
                       is_cylindrical=self.is_cylindrical)
            d0 = s.direction
            d = self.fields.normal_direction(v.swigobj) if d0 < 0 else d0
            c = mp.direction_component(mp.Sx, d)
            v2 = Volume(center=s.center, size=s.size, dims=self.dimensions,
                        is_cylindrical=self.is_cylindrical).swigobj
            vol_list = mp.make_volume_list(v2, c, s.weight, vol_list)

        stuff = add_dft_stuff(vol_list, fcen - df / 2, fcen + df / 2, nfreq)
        vol_list.__swig_destroy__(vol_list)
        return stuff

Note that the direction, d, is available, but stuff.normal_direction is NO_DIRECTION. How can we make this work?