Add communicator interface

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.

1. ParamterTest needs CUDA to compile, even though it is not necessary for the tests themselves
2. The new test from !227 (merged) fails in the pipeline (e.g. https://git.rz.tu-bs.de/irmb/VirtualFluids_dev/-/jobs/137967)

Adding an interface for the communicator fixes both problems. The new interface is named Communicator, while the concrete class was renamed to MpiCommunicator

Note: As mentioned by Sören, the communicator should be reworked and use the existing code in src/mpi (https://git.rz.tu-bs.de/irmb/VirtualFluids_dev/-/issues/45)

src/gpu/VirtualFluids_GPU/Communication/MpiCommunicator.h

@@ -70,8 +70,6 @@ private:
     std::vector<MPI_Request> requestGPU;
     int rcount;
     //////////////////////////////////////////////////////////////////////////
-    double starttime;
-    double endtime;
     MpiCommunicator();
 };