Fully support metamodel versioning

[sstone1]

Metamodel versioning has been introduced, but the entire codebase is hardcoded to concerto.metamodel@1.0.0, and so we can only support metamodels using this specific version. This limits us when making metamodel changes, as it is not currently possible to deliver a new metamodel version into Concerto and have Concerto still process existing metamodels that use the old version.

This feature should extend Concerto such that we can support existing 1.0.0 version metamodels and a new 1.1.0 version metamodel at the same time, as well as defining the strategy for how we eventually support a new 2.0.0 version. Will we support both 1.x and 2.x metamodels at the same time, or will we introduce a new major version of Concerto?