allow providing prefixes declarations separate from list of prohibited within-namespace equivalents

[balhoff]

trafficstars

Prefix declarations are needed for the input TSV. Right now inferred equivalents within these namespaces are also prohibited. @matentzn would like to be able to enable/disable that feature more selectively.

[matentzn]

Yeah, that would be great, in particular to say: MONDO prefix: equivalents not allowed UMLS prefix: equivalents allowed :D

Else I have to do a lot of hacking when we deal with proxy merges in MONDO!