Correct use of caches in the self-PP gravity task. Make sure we can run without vectorization. Make sure we can run without hydro.

Bug fix in the test suite. Move the gravity leaf P-P interaction in the frame of the cells. Use a cache for the self P-P interaction.



See merge request !400

Do the P-P gravity calculations in the frame of the cells for more accuracy in the distance calculations.

Use particle caches for the gravity P-P interactions. Ewald summation for the gravity force checks



See merge request !398

Stop when affinity is requested but not available

Fixes #346 

See merge request !397

Used static arrays for the Ewald correction. Dump table to a file. Correction is within 1e-5 of the Gadget value.

Compute one octant of the Ewald corrections at start-up for future use when running with the gravity checks.

Free sort indices and task arrays before repartitioning

Freeing the sort indices and tasks arrays before repartitioning makes that memory available so 
that we have more room for accepting particles from other nodes.

Part of #98.

Also adds a useful debug function that reports the current memory
usage of the process tree (i.e. call from main thread to get all usage).
Same numbers you get out of the top program.

Free the task arrays and cells are refactored into simple functions to cut down
on cut and paste.

See merge request !392

Also updated the Pressure-Entropy SPH scheme to use the new definition of the number of neighbours and its derivative. Back to correct results.

Lots of refactoring to tidyup code reuse

runner_dopair_grav() should abort if the distance between the cells is larger than the mesh smoothing scale.

Updated version number to 0.6.0 to reflect the large number of changes to the threadpool, drifts, MPI and scaling performance in the last few months.

Periodic gravity speed and accuracy improvements



See merge request !393

Threadpoolize all the things

These are all the remaining function from the list in #338. I've tested this as far as possible on my laptop, i.e. MPI and gravity *should* work.

See merge request !384