WIP: x parity constraints for eigenmodes

[danielwboyce]

Closes #1109.

In process of making changes to cyclically rotate Meep's coordinate systems when passed into MPB so that wave propagation is always in the x direction. So of the math involved and its implementation (notation) is a bit opaque, so at the moment the largest part of the problem is understanding what exactly is happening in the src/mpb.cpp so I don't miss any necessary transformations, either from Meep to MPB or from MPB to Meep.

[danielwboyce]

The direction object d hasn't been rotated correctly; I need to update it with the correct way for direction objects.

[danielwboyce]

At the moment when I run python/examples/mode-decomposition.py with the compiled test version I'm getting an error that seems related to SWIG:

Traceback (most recent call last):
  File "/home/tests/mode-decomposition.no_rotation.testing_version.py", line 61, in <module>
    sim.run(until_after_sources=mp.stop_when_fields_decayed(50,mp.Ez,mon_pt,1e-9))
  File "/usr/local/lib/python3.6/site-packages/meep/simulation.py", line 2222, in run
    self.init_sim()
  File "/usr/local/lib/python3.6/site-packages/meep/simulation.py", line 1321, in init_sim
    self.add_source(s)
  File "/usr/local/lib/python3.6/site-packages/meep/simulation.py", line 1603, in add_source
    add_eig_src()
  File "/usr/local/lib/python3.6/site-packages/meep/__init__.py", line 3947, in add_eigenmode_source
    return _meep.fields_add_eigenmode_source(self, *args)
RuntimeError: meep: invalid dielectric function for MPB

[danielwboyce]

Actually judging the error to be caused by SWIG seems a bit hasty; I'm just not quite sure what's caused it yet.