Make sure that the default constructor sets MPI_COMM_SELF as the communicator.

Before loadbalancing the grid is actually a sequential one with all
the data on one processor. But previously the communicator stored
inside was MPI_COMM_WORLD (i.e. all available processes). This was
counter intuitive as the non-loadbalanced did not communicate with
other processors. To reflect this we now use MPI_COMM_SELF (process
only communicates with itself) and change this during loadbalance.