Modifications in the concept of the different histograms.

In class templates Histograms::HistoBase, Histograms::Histocrete and Histograms::Histogram:
Made it possible to create an empty histogram from an arbitrary other histogram
* Removed the functions get_binning_width() and get_binning_reference() as well as set_binning() from class HistoBase
* Added a function initialise_empty() to each histogram class template, that creates and histogram that has all the x-values of the parameters (with y-values 0) and the additional information for each derived histogram class template (e.g. binning width)
In histogram test classes:
* Added tests for the new function initialise_empty()

In class template WangLandau:
* Included a prototype Histo in the parameter class from which all histograms needed by the algorithm can be constructed.
* Removed the calls to set_binning_width() and get_binning_width()

In class template Observables::HistogramAccumulator
* Replaced the call to set_binning_width and set_binning_reference with a call to initialise_empty when creating the normalized_histogram

Small modications in example program metropolis_histogram to fit the requirements.