ENH: refactor + parallelize the evaluate-fit-classifier and cross-validate pipelines

[nbokulich]

Some re-organization is in order. In particular, some steps could be parallelized so that fitting occurs simultaneously on different folds (for the CV action), instead of serially. The result is quite some inefficiency as this time-consuming step (that cannot be parallelized within a fold) occurs in serial.

An experimental vsearch LOO classifier could also be considered for re-addition in this context. It was removed here:
#130