Clarification of error case behavior when calling ga4gh.vrs.normalize()

[theferrit32]

I'm trying to document the less-obviously-an-error cases for inputs to normalize(), and what the behavior is for each.

Cases which explicitly raise an exception:

• input is not a Pydantic object

Cases which implicitly raise an exception:

• input is a Pydantic object but not a VRS object (or does not have a .type attribute)
• input.location is an iriReference instead of a SequenceReference

Cases which quietly (maybe with a log warning) return the input value, unmodified:

• input.type is not either Allele or `CisPhasedBlock'
• input.start or input.end is a definite ambiguous endpoint (e.g. (1_2)_(3_4), but not (?_2)_(3_?))
• input.state is not a LiteralSequenceExpression