RESS: Add option to return mixing and unmixing matrices

[Hororohoruru]

Pull request adding the changes discussed in #27

The changes are very minimal. I tried to respect the style for the docstring and added also a small example of the potential use of the returned eigenvectors.

Please check whether the end of the function (intending to take care of any combinantion of return_maps and return_comps) is OK or maybe there is a cleaner way of doing it.

[codecov]

Codecov Report

Merging #28 (6a3469d) into master (0676e2f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@ Coverage Diff @@ ## master #28 +/- ## ========================================== + Coverage 75.74% 75.75% +0.01%  ========================================== Files 18 18 Lines 2057 2058 +1 ========================================== + Hits 1558 1559 +1  Misses 499 499 

Impacted Files	Coverage Δ
meegkit/ress.py	100.00% <100.00%> (ø)

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[nbara]

@Hororohoruru thanks !

I've taken the liberty to try and clarify the maps/comps distinction by renaming them to from_ress / to_ress, and expanding the dosctring accordingly.

There is a breaking change wrt to the previous approach, in that I decided that the from_ress / to_ress matrices should have a dimension corresponding to n_keep (previously they were both square matrices). This makes it super easier to project to/from component space.

What do you think ?

[Hororohoruru]

Hello @nbara ! Looks good to me, I think it looks much more clear now. Thanks a lot!