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

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.