Add Migrate for avoiding MPI comms buffer reallocation

[streeve]

Follow on to #549 for particle migration. Extends the same idea to a new Migrate object that can be reused. Still possible to call migrate for backwards compatibility (will still be slow and reallocate a new comm buffer every time)

Also rewrites migrate tests to be more reusable

Still needs:

• commit clean-up
• fixes for some migrate tests
• implementation within ParticleGridDistributor
• Performance benchmark update & comparisons

[streeve]

@weinbe2 just FYI that this exists