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

- Parameter is passed as reference now (according to https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#f7-for-general-use-take-t-or-t-arguments-rather-than-smart-pointers)
- Where possible LBMSimulationParameter is passed instead of Parameter

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.