Documentation update, additional return values from minvar_matrix_make

[jameswilburlewis]

From discussion between Vassilis and one of his students:

Reporting from discussion with student Ryoma Matsuura (cc-ed here).

The minvar_matrix_make in both IDL and Python produce a matrix using the 3 eigenvectors but it is unclear which is the max, int, min, since IDL indexing is different from Fortran and C, and may be different from Python. So currently the user will have to guess or test. Could you comment on how the user can extract the right eigenvectors from this matrix (use an example specifying which index corresponds to which).

Additionally, it would be really nice if the program produced also the eigenvalues corresponding to these directions as an additional variable (max, int, min). Could you make that change and document it (feel free to assign)?

We need these in both IDL and Python SPEDAS.