With the creation of pppm/old, we no longer need PPPM::diffpr and we can change the name of PPPM::rms to make it more meaningful and require fewer variables be passed to it. Minor clean-up.

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@8549 f3b2605a-c512-4ea7-a41b-209d697bcdaa