In merge !227 (merged) a test was added to ParameterTest which needs an instant of the class Communicator.
Using the concrete class poses two problems, as the communicator depends on CUDA.
ParamterTest needs CUDA to compile, even though it is not necessary for the tests themselves
Adding an interface for the communicator fixes both problems.
The new interface is named Communicator, while the concrete class was renamed to MpiCommunicator
