Properties of the self-gravity scheme are now read from the YAML file. They are printed to stdout and to snapshots.

Properties of the self-gravity scheme are now read from the YAML file. They are printed to stdout and to snapshots.

Added the option to replicate the particles along the axis to create artificially larger test cases. Driven by the parameter 'InitialConditions:replicate'.

No unnecessary loop in pair

This implements two tiny improvements:
 
 - In the PAIR task, only loop over the particles in a cell that has active particles.
 - In the ghost task, construct the list of indices of active particles only. No need to loop over the inactive ones.

See merge request !313

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 (Now fixed independently).

See merge request !310