non-oak discovery of EnvO class/subset relations

[turbomam]

needs

• turn the notes form this issue and PR into documentation about integration with EnvO
  • Set up NMDC environmental context subsets with Protege 5.6.4 EnvironmentOntology/envo#1598
  • 294 create branch with content of PRs #285 and #289 for testing value sets/enumerations/subsets integration with EnvO #295
• more standardized IRI<->CURIe handling (not just string substitution)
• print statement about missing label could be an exception raise

Ideally would use oaklib not rdflib