Make frrsa exectuable for n_conditions < 9

[PhilippKaniuth]

Currently, the absolute minimum of conditions that need to be present is 9 (also see known issues).

A "proper" LOOCV could solve this issue. Currently, the lower bound of conditions is 9 only because the algorithm expects at least 3 conditions to be in each test fold (to have more than 1 condition pair in the test fold) in order to correlate the predicted and test dissimilarities. A "proper" LOOCV could deal with only 2 conditions in the test set.

This, however, would entail a major change to the structure of the code base, away from the "correlate predicted with test values directly" approach to an "iteratively fill the predicted RDM with all LOOCV runs to finally correlate the whole predicted RDM with the whole test RDM" - approach.

This is out-of-scope for the time being.