Testing scattering matrices

[GoogleCodeExporter]

Implement Van der Mee & Hovenier test on the Mueller matrix. Probably makes
real sense only for the results of orientation averaging.

Original issue reported on code.google.com by yurkin on 24 Dec 2008 at 7:36

[GoogleCodeExporter]

This also applies to spherical-harmonics expansion of the scattered fields 
(issue 138).

#138

Original comment by yurkin on 4 Jul 2013 at 10:04

[myurkin]

This can also be implemented as a separate tests, e.g. in tests/equiv