Add ability to return covariance matrices from eigenvalue analysis functions.

[willjbrown88]

In magpysv.denoise there are two versions of the eigenvalue_analysis, it would be useful if one or both could optionally return the covariance matrices associated with the eigenanalysis.

I think this is a simple addition to the returns of the usual denoise.eigenvalue_analysis(), and I think the method sklearn.decomposition.get_covariance() is equivalent for the other denoise.eigenvalue_analysis_impute(). The latter would need some testing I expect.

[gracecox]

Covariance matrix now returned by magpysv.denoise.eigenvalue_analysis()