Retain ontology PURL when performing ontology validation

[vedina]

The full validation includes checking for existence of ontologies and ontology entries, through Bioportal and OLS queries. The OntologyValidationManager keeps only boolean cache if the ontology or an entry exists, while it would be useful to retain the PURLs .
This is useful when using the validator programatically, and especially for https://github.com/ToxBank/isa2rdf