Usage of single Communicator class

Currently, our code has two Communicator classes, handling interprocess communication. One is located in src/mpi. It was extracted from the cpu part of VirtualFluids and is still only used by the cpu part. The other one is in src/VirtualFluids_GPU/Communicator and is used by the gpu component. However, both Communicators have a lot of redundant parts. Therefore, it makes sense that the gpu component also makes use of the general Communicator from cpu/mpi. At the moment, this can not easily be done, as the gpu Communicator has additional functionality, which is only used there. Here we have to find a suitable solution to remove the redundancy, but keep the gpu related functionality.