BM and PW plotting re-integrated, but they do not agree on the interface...
plots agree fairly well on either side of interface.
Trying to implement passing of index_pw_inv
PW plotting functioning well, however TE polarisation seems odd..
2D plotting working for both polarisations, as long as off-normal.
The remaining problem lies in different sortings of index_pw_inv between
fortran and python...
Sorted out the sorting issue! Trouble was in the degeneracy of normally incident PWs.
Needed both alpha and beta != 0.
Altered field plotting data structure to allow for catonation of full stack in 3D.
Updated tests for new definition of normal incidence.