Reduce number of all-to-all communications

Attempt to answer #273. Just a place marker for the moment.

See merge request !323

Do not recurse to lower cells within the sub-tasks if they don't contain any particles.

This should fix #274. @jwillis could you please check that your setup that crashes works with this version ? 

It builds on your proposed fix but I moved the decision of aborting higher up in the recursion. 

See merge request !326

Do not recurse to lower cells within the sub-tasks if they don't contain any particles.

This should fix #274. @jwillis could you please check that your setup that crashes works with this version ? 

It builds on your proposed fix but I moved the decision of aborting higher up in the recursion. 

See merge request !326

Added smoothing length correction limiter to Gizmo hydro_predict_extra.

For some reason, the smoothing length prediction in GIZMO sometimes goes completely wrong, causing the code to crash in space_regrid, see #271. To overcome this, I have added a limit on the increase in smoothing length during the prediction step for GIZMO. This appears to solve the issue @jwillis was having with the EAGLE_12 test (I am currently testing it on one of my machines, and it's still running after 6000 steps).

I'm not sure it is worth looking into the cause of the bad smoothing length predictions. The smoothing length update is computed based on a mixture of SPH and GIZMO variables, so this is bound to go wrong sometimes. Maybe there are smarter ways to do this.

See merge request !318