Clarify various object names

[PhilippKaniuth]

frrsa in frrsa/fitting/crossvalidation.py optionally returns the object predictions. This object has the hard-coded column names "dissim_target" and "dissim_predicted". In case of fitting representational matrices that contain similarities, these column names are confusing.

Either conditionally name the columns based on whether similarity or dissimilarity is predicted or use a generic name that'd cover both cases.

[PhilippKaniuth]

Rename func collapse_RDM to sth. that reflects that it can be used for RSMs as well.