Corrected the signature of the incremental operators in the histogram class templates, now returning an reference to the histogram instead of void.
Changed the organisation of the histograms:
* Deleted the files histogram_number.hpp and histogram_number.cpp that contained an histogram only for numerical x-values
* Inserted a class HistogramNumber in file histogram.hpp that inherits from the general Histogram and sets the BinningFunctor correct
* Inserted a class HistogramVector in thile histogramm.hpp that inherits from the general Histogram and sets the BinningFunctor correct. This class does not work at present, because std::vector does not support the operator<< for output.
Modified the Histograms:
* Made the get_binning_width and the get_binning_reference functions const
* Modified the constructors to take HistoBase objects
Changed the normalizing procedure of HistogramAccumulator (now using set_binning_width and set_binning_reference instead of set_binning)
Smaller modification in the tests of histogramm_accumulator.
Added a function derivative to class HistoBase that calculates the derivative of a histogram with respect to its x-value.