Added function HistoBase::count_y to count the number of Histogram entries with given y-value.
Added the first version of optimal ensemble sampling according to Trebst and Troyer.
Extended the documentation of the class template OptimalEnsembleSampling.
Inserted the signal handlers in class template OptimalEnsembleSampling.
Corrected to minor typos and errors.
Added function HistoBase::compatile() to check whether all x-values of the one HistoBase are also contained in the other histobase.