Moved Simulation::init() to Constructor, Simulation:free() to destructor and exchanged some members of Simulation with unique_ptr instances.

return std::optional from findIndexInSendNodes

> rule of zero

A Communicator is now passed to IndexRearrangementForStreams (Commit #7e1ef552). In the constructor of this Communicator MPI_Init is called. Therefore, an additional call to MPI_Init is not needed anymore.

A Communicator is now passed to IndexRearrangementForStreams (Commit #7e1ef552). In the constructor of this Communicator MPI_Init is called. Therefore, an additional call to MPI_Init is not needed anymore.

Some variables were not assigned values in the constructor.  This caused an "MPI_ERR_COMM: invalid communicator" error when using multiple gpus.