Unify IdCounters
The struct 'multigrid' (in gm.h) contains five counters:
INT vertIdCounter;
INT nodeIdCounter;
INT elemIdCounter;
INT edgeIdCounter;
INT vectorIdCounter;
These are used to produce the 'id' fields in the respective objects (also in gm.h).
However, these id fields are nowadays really only used for the dune-grid IdSets (this needs to be checked). Those want a single id for all entities, not one for each codimension. The five counters could then be replaced by a single idCounter.