Star particles and gparts links over MPI

A big bundle of changes:

• Introduce star particles. At the moment they are empty shells but they are ready to be used.
• An spart is always linked to a gpart and will receive its gravitational forces from there.
• A gpart now contains an enum containing information about whether it is linked to a part, to an spart or to nothing.
• Added a more comprehensive test to check that the part<->gpart and spart<->gpart links are correct after every rebuild or MPI transaction.
• spart get kicked and drifted and get their time-step computed.
• spart are read-in and written to snapshots (only if running with -S for now).
• Only the particle types that will be used are now read from ICs. Speeds-up the starting up process.
• Corrects many incorrect communication of gpart over MPI.

With this in we can run the EAGLE_25 over MPI with gpart and spart activated (e.g. swift_mpi -s -g -S -t 16). Note that this does not fix the remaining bug in #256 (closed) (Now fixed independently).