Altered the MetropolisParallel::do_parallel_metropolis_simulation() method: Now constructing the copied configuration spaces and the new simulations in the for loop and not before. This can lead to less memory consumption, especially if the number of runs is greater then the number of processes to use.
Added operators for adding and substracting a scalar to and from an ArrayObservable and VectorObservable.
Added tests for the new operators.
In functions Metropolis::autocorrelation_function() and Metropolis::integrated_autocorrelation_time():
* Added support for non-trivial observable types
* Added a test for non-trivial observable types

Both functions still have to be tested for correct return values.
Added const-ness to some get-accessors in HistoBase, HistogramNumber and Binning and to the sum-function in HistoBase.
Fixed the assignment operator in class ArrayObservable.
Added a class template HistogramAccumulator for accumulating observables in an arbitrary histogram. Will mainly be used with the histograms defined in namespace Mocasinns::Histograms. Added tests for the HistogramAccumulator.