Enable overriding default OLS and BioPortal URI construction

[cthoyt]

As a follow-up to #569, it's the case that several ontologies in the OLS don't use OBO PURLs, which means the automated code for constructing OLS (and BioPortal) URIs is broken.

E.g., for KISAO, the term with URI http://www.biomodels.net/kisao/KISAO#KISAO_0000057 gets the following:

• OLS: https://www.ebi.ac.uk/ols/ontologies/kisao/terms?iri=http%3A%2F%2Fwww.biomodels.net%2Fkisao%2FKISAO%23KISAO_0000057
• BioPortal: https://bioportal.bioontology.org/ontologies/KISAO?p=classes&conceptid=kisao:KISAO_0000057

[StroemPhi]

Same issue for the nmrCV (former NMR). The correct example links would be:

• OLS: https://www.ebi.ac.uk/ols/ontologies/nmrcv/terms?iri=http%3A%2F%2FnmrML.org%2FnmrCV%23NMR%3A1000003
• BioPortal: https://bioportal.bioontology.org/ontologies/NMR/?p=classes&conceptid=http%3A%2F%2FnmrML.org%2FnmrCV%23NMR%3A1000003