Handle BAG OF Enums attribute definitions in 2.4

[signedav]

Since INTERLIS 2.4 it's allowed to do BAG OF AttrType defined on attributes:

    CLASS MaterialClass =
      Material : ( Straw, Wood, Stone );
      BagOfMaterial : BAG {0..*} OF (Concrete, Clay, Plastic);  
    END MaterialClass;

Material : ( Straw, Wood, Stone ) is handled since , but BAG {0..*} OF (Concrete, Clay, Plastic) is not.