`prov:Derivation` is not a `prov:InstantaneousEvent`, so should be handled separately

[ajnelson-nist]

I noticed this code block:

CASE-Implementation-PROV-O/case_prov/__init__.py

Lines 257 to 263 in 8441e4b

	n_instantaneous_event_type = {
	NS_PROV.qualifiedCommunication: NS_PROV.Communication,
	NS_PROV.qualifiedDerivation: NS_PROV.Derivation,
	NS_PROV.qualifiedGeneration: NS_PROV.Generation,
	NS_PROV.qualifiedInvalidation: NS_PROV.Invalidation,
	NS_PROV.qualifiedUsage: NS_PROV.Usage,
	}[n_predicate]

Double-checking prov:InstantaneousEvent, there are only five subclasses in PROV-O:

• End
• Generation
• Invalidation
• Start
• Usage

Derivation should be separated from that highlighted code.

[ajnelson-nist]

On reviewing the code, it was not a bug, and documentation may help clarify the intent. #136 posted.