Create functions for mixing matrix application

[ntessore]

Create a set of functions that takes (potentially binned) mixing matrices and applies them to an theory power spectrum, producing (potentially binned) spectra with the right columns.

[ntessore]

This might be better off-loaded to e.g. euclidlib.

[ntessore]

This would be much easier to implement if #211 was merged, since the full set of mixing matrices and spectra for one bin are then contained in a single array.