communication_mpi.c is now compatible with main REBOUND branch
particle.c now more in sync with main REBOUND branch. New function
..._not_tree needs to be updated
particle.h more compatible with main REBOUND version. need to fix
..._not_tree function.
Merge branch 'erika' into SMACK-hanno
Removed particle_add_local_not_tree() function
Added FRAGMENTATION_SMACK to makefile