Broke one of my own rules by doing too much in a single commit: implemented runner_extra_ghost, moved gradient calculations for both gradient types to new gradient loop, fixed bug in engine.c involving the number of links (which is larger with an extra loop), streamlined gradient methods, documented most GIZMO specific files. Second loop works.

First initalisation of particles now happens in space_init() and not in engine_init_particles() any more.

Add entropy to the energy diagnostic file. Use an MPI_Reduce instead of MPI_Allreduce to get the values

Renamed 'h' in cell struct to 'width' and renamed 'h' and 'ih' in the space struct to 'width' and 'iwidth'.

engine_step now writes to a file called timesteps.txt that contains timestep information along with a header.

the super cell is either a non-empty leaf or the first cell for which an interaction task is defined. need to add a condition for gravit tasks once these have been settled.