added Environment for MPI initialization

This MR replaces the old AMDiS::init() - AMDiS::finalize() pattern with a RAII class Environment, to be instantiated at the begin of the main method, i.e.

int main(int argc, char** argv)
{
  Environment env(argc, argv);

  // get MPI rank and size from environment after instantiation:
  std::cout << Environment::mpiRank();
  std::cout << Environment::mpiSize(); 
}

the ``finalize'' is called automatically when env is destructed, i.e. at the end of main().