Corrected some typos in array_observables.
Included an internal member rng_seed and the get_accessor get_random_seed in class simulation. Provided a standard value in the two constructors of the class template Simulation.

For parallel algorithms one likes to start from multiple seeds and needs to know the present seed of the random number generator.

The provided random number generator need not to provide the possibility to access the seed, so it has to be stored in the Simulation.
Included namespace identifiers in the include guards of all files.
Moved the vector_accumulator to a seperate details file so that it can be used by other algorithms.
Added a parallel (using openmp) Metropolis algorithm and tests.
Code reorganisation: Only declare sub-classes in the main classes and define them after the code body.

This was applied to the parameter classes in the class templates Metropolis, MetorpolisParallel and WangLandau as well as for the structs in WangLandau used in signals.
Code reorganisation: Made the Parameters of the Metropolis and the parallel Metropolis simulation to member variables and removed it from the single functions (as it was allready before for the Wang-Landau Simulation).