Combine symbolic and concrete structure in wrapper

[Broes De Cat]

Currently, a.o. grounding has to keep track of both structures and remember to query both of them to find out the value of an atom/function. It would be less error-prone to write a wrapper around them in which both are always queried. For example know, the lazy grounding has to keep track of the symbolic structure because it is in general too expensive to first expand the concrete structure with the new symbolic information.