unify random number generators
In Base/Math/IntegratorMCMiser.h we are using gsl_rng; elsewhere std::rand or std::default_random_engine, and !2672 (merged) introduces std::mt19937.
In Base/Math/IntegratorMCMiser.h we are using gsl_rng; elsewhere std::rand or std::default_random_engine, and !2672 (merged) introduces std::mt19937.