Implement unified behavior for owl:imports

[cmungall]

Currently different libraries have different behavior for owl:imports

• rdflib ignores
• pronto follows by default, controlled by import_depth
  • doesn't use catalog-v001.xml? Add supports for catalogs in imports althonos/pronto#186
  • will fail if any ontology in the import chain is in something pronto cannot parse, e.g. functional
• funowl ignores
  • even if implemented, unless more parsers are added to funowl, will fail if any ontology is import chain is not functional
• owlapi (not used by oak except via robot plugin) follows by import
  • respects catalog-v001.xml
  • has complex customizable behavior
• remote endpoints (ols, bp, ubergraph) typically merge imports in advance
  • may be stratified in different graphs
• rdftab ignores
  • but in semsql imports closure typically pre-merged