Modified the test makefile to deal automatically with all test files.
Modified addition and substraction of VectorObservable:
If one adds or substracts an empty VectorObservable, the other VectorObservable is unchanged, and no exception is thrown. Now a VectorObservable can be used without initialising in a boost::accumulator.
Modifications in class template VectorObservable:
* Made it possible to multiply the VectorObservable with an arbitrary scalar
* Corrected the assignment operator.
Added example simulations combining the Ising classes and the Mocasinns::Metropolis simulations. Further examples will follow.