Moving towards support for multiple languages

[Jörg Kienzle]

The goal of this issue is to move to the new metamodel that will support multiple languages, and also treat models as completely external.

[Matthias Schoettle]

References #509: Fixes OCL constraints to use RAM enum types instead of CORE ones.

→ <<cset cf06762815a7>>

[Matthias Schoettle]

References #509: Adds workaround to Validator to also check the artefact.

Checks the external artefact related to the given model. This allows constraints to be evaluated that correspond to one of the three interfaces.

→ <<cset 172ce75b569e>>