2.2.0

Added

• threads parameter to Ontology constructor to control the number of
  threads used by parsers supporting multithreading (OBO and OBO JSON at
  the moment).
• Deprecation warnings for suspected uses of the is_a pseudo-relationship
  since subclasses/superclasses is now to be handled by the owner Ontology.
• Support for subclass/superclass edition directly from the objects returned
  by Term.subclasses() and Term.superclasses(). (#84)

Changed

• Updated fastobo to v0.8, which reduce memory footprint of identifiers,
  and improves the parser speed.
• Improved OBO parser performance using threading plus zero-copy validation
  of identifiers on Xref instantiation.
• Improved performance in debug mode by having the typechecker only extract
  the wrapped function signature once.

Fixed

• OBO parser crashing on files containing idspace clauses in their headers.
• Reference management issue with binary operations of TermSet.

Removed

• nanoset depency, which was not useful anymore in Python 3.8 and caused
  issues with multithreading when processing OBO frames in parallel.