Merge branch 'Gravity_basics' into 'master'

Basic implementation of gparts

That's a rather big one, sorry. 

It implements the treatment of gparts everywhere in non-MPI land. Particles can be read in, and written to a file. Hydro particles get their gpart doppelgänger automatically created and propagated to the correct cells. 

This merges in part of @tt and @jregan's work of the last few weeks

No gravity-related tasks are implemented for now. 

In MPI-land, the gparts are not created (and not read in) as some crucial piece of code is still missing.

Nothing should break on the normal test-cases but we can now start playing with gravity on one single node.

The merge request is rather large as it involves changing all the IC scripts to always have IDs>0

See merge request !116