Make global topology const in ISimulator

The global topology is constant in all simulator level functions with the exception of the setup of MiMiC runs and reruns. Follow-up work will move the topology-altering MiMiC setup out of the simulator level functions. It is, however, worth to already enforce the current state, avoiding subsequent changes to undermine these efforts. This commit changes the pointer to the global topology in ISimulator to be const, and casts the const-ness away for the two known cases it needs to be non-const.

Refs #3467 (closed)