Sanity check @Interface annotations

[szschaler]

Should add a validation that sanity checks any interface_of statements to ensure the use of @Interface annotations is sensible. For example, where a reference is annotated as @Interface, it makes sense to expect that the classes at either end are also annotated in this way...

[narudocap]

I'm not sure a sanity check on the metamodel would be enough. The issue #32 was related to rules and identity rules. Actually, if I use "virtual" instead of "virtual identity" in the mapping something gets broken:

[szschaler]

I think that error is actually separate: what happens here is that the name of the mapping being referenced from the weave doesn't actually exist. (if you get rid of the hover text, you will see a wriggly red line under it.) This leads to the above follow-on error. Not ideally, certainly, that this raises an exception.