Added the function SpinLattice::system_size() to easily calculate the size of the system. Added test functions.
Added two functions to the Metropolis simulation class template to calculate (1) the autocorrelation function for an observable and (2) the integrated autocorrelation time for an observable.

The functions still need to be tested, especially with VectorObservable and ArrayObservable.
Implemented the possibility to set the number of processes in parallel metropolis.