Add ability to filter ontology classes when performing analyses such as enrichment

[cmungall]

E.g. remove all cell component; remove all with a particular subset (antislim)

request from: @putmantime for Translator QC notebook

[cmungall]

Actually now I look at the code again maybe not necessary.

    def enrichment_test(self, subjects=None, background=None, hypotheses=None, threshold=0.05, labels=False, direction='greater'):

TODO: document hypotheses - this is simply the list of ontology class IDs tested. If empty uses whole ontology, but for full control pass your own IDs here