Major changes in in the histogram classes:
* Applied the CRTP (couriously returning template patern) to class HistoBase: Teh class template has now a third template parameter for the derived class.
* The new template parameter is used to define the arithmetic operators +=, -=, *= and /=, as well as +, -, *, / in the class template HistoBase (it uses the operator[] and the insert of the derivative class and returns the derived histogram)
* Applied this also to the save/load_serialize/csv functions

Advantages: The code for the arithmetic operators and for serializing needs now not to be repeated in every derived class.

ToDo:
* Implement a possibility to convert Histograms with different y-value-types (in the constructor), e.g. for dividing a Histo<...,int> by an int, but for floating precission.
* Implement operators like pow, sqrt ... acting binwise on the histograms.
* Spezialise the load and save csv functions for x-value-types vectors, array ...