ENH: Remove legacy #ifdef USE_VTK_COSMO
ENH: Remove MPI_COMM_WORLD from InitialExchange

Introduce an internal ivar, Communicator, that the client can set
such that this code can use a user-supplied communicator. By default,
this->Communicator is set to MPI_COMM_WORLD in the constructor. In all
other methods when a communicator is needed this->Communicator is used
instead.
ENH: User-supplied communicator for Timings::print

Added optional parameter to Timings::print s.t. a user-supplied
communicator can be provided. If no parameter is provided,
comm=MPI_COMM_WORLD.
ENH: Get comm from Partition::getComm method

InitialExchange should be called after Partition::Initialize to ensure that
a the communicator is supplied.
ENH: Supply communication to Partition::initialize