Make global topology const in ISimulator

Make global topology const in ISimulator

The global topology in 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