CSL/CTMC Simulation: Improve speed for the Steady-state operator

[ivan.zapreev]

When doing regenerative simulations, for example taking the test case

test/functional_tests/ctmc/csl/simulation/time_unbounded_until/csl_unbounded_until_sim_06

and the property

S{>0.94}[ fq ]

with initial state 20201 the regeneration cycle needs an enormous time to be complete. Therefore we have to add a feature that will allow to choose a regeneration point that is visited rather often. The following might be wrong from the simulation point of view, but for example:

1. We take the BSCC that we are to simulate
2. We simulate a trace of the length 2*|BSCC| (the number of BSCC states)
3. We choose the most often visited state in this trace and make it the regeneration point

Also we might want to use chain-splitting method which is supposed to speed up the regenerative simulations.