Class VectorObservable:
* Added tests for scalar multiplication and division
* Overloaded the function pow for use with VectorObservable and scalar (component-wise power)
* Added tests for the pow-function.
Modifications in the organisation of the Unit-tests:
* Gathered all test files dealing with histogram in a subdirectory of tests
* Adapted the testprogram and the Makefile to deal with the new subdirectory
* Annotated the test structure in the output of the test functions.
Modifications in the organisation of the Unit-tests:
* Gathered all test files dealing with observables in a subdirectory of tests
* Adapted the testprogram and the Makefile to deal with the new subdirectory
* Annotated the test structure in the output of the test functions.
Added another observable (based on std::array) with the necessary functionality. Added tests for the ArrayObservable.
Modified VectorObservable and ArrayObservable to be able to use them in an boost accumulator.
* Added operator * and / for multiplying and dividing two VectorObservables and ArrayObservables component wise
* Added a global function sqrt to calculate the square root of a VectorObservable and an ArrayObservable (this uses the global pow function).
* Specified in the boost namespace how to divide the VectorObservable and the ArrayObservable by an integer (in averaging).
Added functions to the metropolis algorithm to use accumulators instead of returning all the measurments in a vector.

The user can now decide which statistical quantities are important for him. The memory needed for storing the simulation results can thereby be reduced dramatically.
Modified the Doxyfile to also include documentation of files in subdirectories.