Create traceability attributes more efficiently

[BentleyJOakes]

As of commit a422784, traceability attributes are generated for all apply classes when the transformation is exported to DSLTrans classes. (These traceability attributes are the ApplyAttributes used for backward link matching).

There are two improvements to be made:

1. The actual transformation in MPS is modified in this process. It outputs errors on the command line. A better way would be to directly add the traceability attribute to the DSLTrans class being generated.
2. Instead of generating the attribute for all apply classes, we should instead figure out which apply classes are connected to backward links, and only generate traceability attributes for those classes