First implementation of gpart motion

The changelog is rather big but the changes are rather straightforward:

 - The acceleration of the `gparts` is now called `a_grav` instead of just `a`. Makes grepping through the files much easier.
 - I have added the command-line options `-g` and `-G` for external- and self-gravity respectively. These build the correct policies into the engine. Separating both (at least for now) allows to work on the two aspect separately.
 - There are tests in critical places to make sure that linked pairs part-gpart are at the same position.
 - The `doinit()`, `dodrift()` and `dokick()` tasks have been updated to also move the gparts.
 
All the tests pass successfully. I am seeking your opinion on the implementation of the tasks. 

See merge request !135

Updated vectorisation tests



See merge request !134

Updated vectorisation tests



See merge request !133

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