Adding in 2 FFT version of PPPM for better scaling. This also includes some modifications to the 4 FFT version of PPPM causing the PPPM initialization to run slower, but loop time is essentially the same.

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@8479 f3b2605a-c512-4ea7-a41b-209d697bcdaa