Modified the Metropolis simulation:
* Included the ConfigurationType::selection_probability_factor() into calculating the acceptance probabilities
* Created the subclass Metropolis::Paramters that stores the relaxation steps, the number of measurements to take and the interval between two measurments.
* Included two functions Metropolis::do_metropolis_simulation for one and for a range of temperatures that perform the measurments.
Added a class "VectorObservable" that can be used for storing a vector of different observables in the Metropolis and future simulations. Added a test class for testing VectorObservable.

VectorObservable contains a std::vector as data member and forwards all functions that are defined for a std::vector. Additionally, one can perform addition and substraction of VectorObservables and scalar multiplication (these capabilities are needed for averaging observables).