Interpretation retrieval is dangerous when storing it

[Broes De Cat]

It returns the local copy, so changes are propagated to the structure, but if the structure's vocabulary changes and it is deleted, it results in dangling pointers. So should verify which should use new tables and which can use the local ones, and decide on the semantics when changes are applied.

Seems like a good case for shared_pointers.

refs https://bitbucket.org/krr/idp/pull-request/77/retrieving-interpretations-from-a