Deprecate RSA functions

[PhilippKaniuth]

There is a well-maintained reference implementation for Representational Similarity Analysis, the rsatoolbox. I therefore suggest to deprecate THINGSvision's RSA functions.

Advantages:

1. End users don't get confused as they only have to use a single gold-standard package for DNN feature extraction and one for RSA analyses.
2. THINGSvision avoids getting issues raised asking for RSA functions currently not implemented in THINGSvision, e.g. other distance functions.
3. Less code for THINGSvision to maintain as the focus is solely on feature extraction.

Possibly PR THINGSvision's RSA functions to the rsatoolbox if they turn out to be more efficient.