Engine exchange strays

Tested this locally with and without MPI. Didn't actually test sending around gparts, because we can't really deal with them quite yet.

See merge request !117

Conflicts:
	src/engine.c
	src/engine.h
	src/part.c
	src/part.h
	src/proxy.h
	src/scheduler.c

This reverts commit 4ab23609

Redistribute fixes



See merge request !131

Redistribute fixes

Fixes #130 and makes `engine_split()` work for gparts.

That lead to some issues in common_io.c but the master works if you don't run with -g. (And -g does not work beyond one time-step anyway as we don't have the MPI communications for gparts yet.)

See merge request !130

Redistribute gparts

Initial version of `engine_redistribute()` which sends around the gparts as well as the parts + preserves their links.

@nnrw56 what do you think ?
I'll need to update some bits later to take into account your changes to the MPI_types and the new version of `space_part_sort()`

See merge request !124

New unit test computing the pair interaction between two neighbouring cells with particles slightly displaced from Cartesian grid.

Rewait fixes

Fixes the deadlock described in #125.

See merge request !122