How to reference internal and external protocols (and possible other types)

[objectiser]

Currently the model reference has a property 'isInner' to determine whether a derived protocol reference is inner or external. At the moment there is no mechanism for determining this - so it is just set to true by default, so that the link to inner protocols can be managed during projection.

Need to decide how this should be determined. Possibly as part of the initial validation of the global model before projection? - although not really validation (which should just be readonly task). May be it could be a final initialisation step on the model, but that would then not be extensible easily. So some kind of binding round might be appropriate.