Add GTS pivot layer

[szschaler]

The tool currently only supports Henshin-based GTSs. We should add a pivot layer to enable support for arbitrary GTSs. I have made some start on this already, but had to move it out for now as the EMF/Xtext wrangling got too much and distracted from the main purpose of the tool.

The exploration/discussion has provided some insights into how to get this done, though. The core idea is to do something similar to what Ed Willinks did for his OCL Pivot model: this is a separate model in a separate resource. The URI for this resource is of the form *.x.oclas where x is an extension specific to a particular language. A special resource factory creates resources for oclas files and these translate on first load. Otherwise, they're pretty much normal resources. The only thing that remains to do then is to get Xtext to load the oclas resources. I was originally thinking of fiddling with how Xtext finds resources and to create virtual resources for all resources to be wrapped. I now think there's actually a much easier way by asking the user to explicitly import the pivot resource into their morphism specification.

Links

• This thread on the Xtext forum has the discussion of some of my ideas
• Branch feature/behaviour_mapping_resource_factory_approach has the latest state of my developments in this direction (not currently functional and not supporting the latest tool features)