The use of --seed in runThePEG didn't work for MultiEventgenerator
since all object get re-initialized for every subrun. This is fixed by
also setting the Seed interface if a --seed is given. In
RandomGenerator the saved Gaussian number is now discarded on
flush(). In MultiEventGenerator, an optional separate RandomGenerator
can be given to generate interface values for each subrun, to ensure
reproducibility of interface values.