Support for metamodels and model types' attributes

[tdegueul]

Cross-references between different models is only in-the-small, at the Ecore level. However, it would be relevant to allow direct references between models by enabling the definition of metamodels' and model types' attributes or references:

modeltype MyMT { ... }
metamodel X implements MyMT {
    ecore "..."
    exactType XMT
}
metamodel Y {
    ecore "..."
    exactType YMT
    attribute myX : XMT
    attribute myRef : MyMT
}
transformation foo(Y y) {
    y.myX = ...
    y.myRef = new X
}

This attributes must be taken into account in the model-oriented type system.