[Parent Issue] datamodel: still todo

[ppanero]

TODO as outcome of #49:

• Implement versioning (conceptrecid minting)
• OAI serializing/schema, add to tests and fixtures.
• Customization of enums
• Resource types are loaded from a JSON file. We need to converge on how to load them, for all enums.
• IdentifierScheme could be reused in many other schemas. However, there is no easy and clean way to flatten its attributes. Tested Pluck and Method. The latter worked but it has a more difficult code comprehension, which in my perspective makes is not a good choice.
• Tests for access_condition since the access part is not fully defined yet. It is not added to the fixtures either.
• Tests for FilesSchema, since is dump_only (see Zenodo). How should this be tested?
• Access: Combine / disambiguate _access and access_right  #37

[fenekku]

Breaking this up into smaller issues and closing:

• Implement versioning (conceptrecid minting) -> global: implemente record versioning #59
• OAI serializing/schema, add to tests and fixtures. -> Implement OAI-PMH serialization #64
• Customization of enums e.g. resource types -> customization: resource types #5
• IdentifierScheme -> Reuse IdentifierScheme #65
• access_condition -> Add access_condition to data model #66
• Serialization of FileSchema -> Serialize FilesSchema (dump_only) #67