QVT14 — Multiple Extents per TypedModel

QVT13-26 requested support for collections of input models. This was resolved by generalizing the type of a ModelParameter to allow a Collection. This change was not necessary.

If each input model has a distinct Extent, there need be no prohibition on the passing of multiple Extents to a single TypedModel. By default this will be treated as a multi-rooted model.

In QVTr/QVTc where Extent is currently unused, reification of Extent as a transformable model element solves problems with providing a 'container' for multi-rooted models and allows collections of input/output models per TypedModel to be managed.

In QVTo, Extent can be similarly reified but care is required to separate the similar functionality of Model as the/one-of-the TypedModel instances.