Merge branch 'gravity_multi_dt' into 'master'

Gravity multi dt

This fully implements self-gravity bar the periodic task over MPI. 

Could you please check that is does not break anything on the hydro side ? 

If you are interested in running with self-gravity I'd recommend using the updated EAGLE_12 case and run with `-D -G`. 

My next steps is to work on the formal requirement of using `-D` alongside the `-G`. I also need to finish working out the automated scripts to generate code for the higher-order multipoles but that is orthogonal to the integration of the code into the master.

See merge request !324