Enum problem

Enums can be generated but not as accurate as possible: The code from the example from https://github.com/FreeOpcUa/opcua-asyncio/blob/master/examples/server-custom-structures-and-enums.py does not work in the generated server, hance programming the generated enum that way doesn't work either. That's why at the moment the varianttype of the enums is ua.VariantTypes.String and only the objecttype is the enum type. This leads to problems since at the moment one could choose a value outside from the enum values as new value as long as its a string.