Figure out how to serialize enum-like classes into schema.org DefinedTermSets

[apirogov]

Instead of simple Literal or Enum types, it might be a good idea to provide a custom serializer with semantic info.

• it should probably be parsed with custom pre-processing (like QuantifiedValue) from string into object
• somehow the enum definition itself must be embedded non-redundantly in jsonschemas (when a value is used, the class must be added to the schema as well)

for the second point the schema normalization could help.

[apirogov]

Maybe not needed after completing https://github.com/Materials-Data-Science-and-Informatics/metador-core/issues/75