Resolve Xrefs equivalence

[omaus]

This PR

• incorporates ControlledVocabulary library
• adds to and shifts functionality from DBXref module to its record type
  • such as the parsing function is now ofString
  • and new toCvTerm functionality
• adds a function to return OboTerms as CvTerms
• adds two functions to OboOntology
  • check for equivalence between two terms using the given ontology
  • getting all equivalent terms between two ontologies
• has unit tests added for everything above

[omaus]

Requests are done, ready for rereview! 😃 @kMutagene