Recover missing mpi_type definitions, remove unused variables, remove repeated drift flag, correct HAVE_MPI to WITH_MPI

* Alternative: does .../thread_siblings contain more than a single 1?
  n.b. May or may not contain a comma if so.

* This is not vectorisation-specific.
* There may be better trade-offs between HT and NUMA.
* Maybe print a warning when we require multiple NUMA nodes.
* Must also detect when Hyper-Threading is not present.
* Probably better as a configure flag, rather than conditional only upon
  the availability of libnuma.
* ~15-40% performance improvement on COSMA.

Created an MPI type for the particles. This is useful for particle exchanges when redistributing. Up to 2^31 particles can now be exchanged at once.