Adding changes to 2 FFT PPPM so that it runs faster in NPT mode. This is due to computing self-force pre-coeffs once that used to be computed after volume changes. These pre-coeffs are not dependent on box size/shape.

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@8762 f3b2605a-c512-4ea7-a41b-209d697bcdaa