Support for enum mapping for slots with multiple enum ranges

[martinwellman]

This PR adds support for mapping slots that have a range consisting of multiple enumerations.

Some important things to review:

1. In object_transformer.py line 186, source_type (of type ElementName) is converted from a string with yaml.safe_load. The expected loaded value is a string (eg. 'myenum') or list of strings (eg. ['myenum1', 'myenum2', ...]). Is this the preferred way to do this?
2. In object_transformer.py in function transform_enum: while iterating over the enum derivations, the first time we encounter a derivation with mirror_source=True we return str(source_value) (if no mapping has been successful yet). Should we instead iterate over all enum derivations, try to map the source_value (ignoring mirror_source), and only after failing all enum derivations should we return str(source_value) if any of the enum derivations have mirror_source=True?

Closes linkml/linkml#2128

[martinwellman]

Should add unit tests.