Fix memory leaks in Relations{1,2}D (follow up !2572)

closes Rec#145 (closed)

Disentangle / distinguish between the pure virtual functions required by IInterface and DataObject, caused by the fact that Relations inherit from both of them -- this resolves the conflict in reference counting required by both base classes, but which of course should happen exactly once.

In addition, add tests, and perform some modernisation.

must be applied in conjunction with Phys!726 (merged) and Analysis!634 (merged)