Gravity p2m improvements

src/Makefile.am

src/cell.h

src/gravity.c

src/gravity/Default/gravity_iact.h

src/gravity/MultiSoftening/gravity_iact.h

src/gravity/Potential/gravity_iact.h

src/gravity_cache.h

src/gravity_derivatives.h

src/kernel_gravity.h

src/kernel_long_gravity.h

src/multipole.h

src/multipole_struct.h

src/part.c

tools/plot_gravity_checks.py