Better evaluation of cluster T-matrix

[johnaparker]

The cluster T-matrix can be computed from a solved miepy.cluster or miepy.sphere_cluster

Attempt at implementation

https://github.com/johnaparker/miepy/blob/issue_2/miepy/tmatrix/cluster_tmatrix.py

Notes

• miepy.sphere_cluster_particle is currently implemented using NFMDS, but would be more general implemented in this way
• The T-matrix can be constructed column-by-column by solving the cluster coefficients for each possible VSHW function as an incident source.

References

Xu.-.2003.-.Radiative.scattering.properties.of.an.ensemble.of.pdf
Eqs. (49-53) for the cluster T-matrix

[johnaparker]

In d9da4bf, there is now miepy.sources.vsh_source, which can be used to construct a column of the T-matrix in combination with the cluster coefficients