MPIdata should be a singleton

[alecjohnson]

I want to check in some changes that make the MPIdata class a singleton. It is possible that in the future we will want to create multiple instances of the iPic3D solver class in a single process, whereas MPIdata needs to be a singleton by virtue of the fact that MPI_Init(), which is called in the MPIdata initializer, should be called only once. A crucial benefit of this change is that the mpi rank is make available to system-level diagnostics code (debug and error messages).