coupling coefficient calculations (only in Mirror currently).

Using np.array instead of np.matrix is advised by the numpy/scipy developers as
the latter could be deprecated/removed eventually; and the former has support for
convenient matrix multiplication as of Python 3.5.

Adding a new code structure for computing coupling coefficient matrices (in Mirror) which
should make it easier to fit together once we have knm_map and knm_aperture matrix fields
in this class. It may also make sense to add symbolic knm fields to the new MirrorKnmMatrices
data class when attempting to implement analytic coupling coefficient computations via sympy
(or otherwise) in the near future.