Enforce correct objectFlow in/within CompoundActivities

[david-istvan]

Connecting objectFlows to/from CompoundActivities has been supported since #35. Ensuring the correctness of the model, however, is deferred to the user.

Implement a correctness enforcement mechanism, outlined in #35:

As a design decision: whenever an object is linked as an input/output to the CompoundActivity, at least the same type of objects should be present during the first/last activity of the typing process. But ideally the same objects could be shown there, potentially through an alias mechanism.