Add a `minimal` module

[cmungall]

See:

• Proposed new module type: minimal INCATools/ontology-development-kit#1352
• Does taxslim contain a class entry for every NCBITaxon purl? #121

It would be useful to have a version of NCBITaxon with only label and is_a axioms (ncbitaxon contains no existential restrictions). This would cut down on the size.

Use cases include

• linkml dynamic valueset checking
• basic propagation scenarios
• some forms of OWL consistency checking (note that for GO/UBERON/CL TC checks more complex axioms are required due to limitations of OWL)