Minimize the life cycle of MPI Comm and PFFT Plans, ProceMesh, etc.

[rainwoodman]

The current life cycle is tied to the ParticleMesh object, causing issues with Python GC and running out of communicators.

Assuming plans are fast (since we do estimated plans) to generate, and mpi comms are fast to spawn we shall be able to minimize their life span to r2c and c2r.