MDSL generator: support enums

[stefan-ka]

MDSL as well as CML support enums, but they are currently not created by the MDSL generator.

--> Add enum support to the MDSL generator.

[stefan-ka]

Solved. MDSL generator creates data types (not P types as so far) for enums now.

Example:
In CML you can model enums and reference them in other objects:

enum TestEnum {
  VAL1, VAL2
}
Entity Address {
  String street
  int houseNumber
  - TestEnum enumVal
}

This results in the following data types in MDSL now:

data type TestEnum {"VAL1"|"VAL2"}
data type Address { "street":V<string>, "houseNumber":V<int>, "enumVal":TestEnum }