(cherry picked from commit 8d9737b04df5158d512b7708023af0b154899fa9)

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@15508 f3b2605a-c512-4ea7-a41b-209d697bcdaa

- remove unused or unneeded class members
- make the code compatible with per-atom masses
- test for and abend in case of an invalid group mass

(cherry picked from commit e017b3389839ff45bc4b5c24c2c7eab1522ff528)

Submitted by Steven E. Strong via github
Contributing authors: Steven E. Strong and Joel D. Eaves   Joel.Eaves@Colorado.edu

This branch implements Gaussian dynamics (GD), which is a method to do
nonequilibrium molecular dynamics simulations of steady-state flow. See
http://dx.doi.org/10.1021/acs.jpclett.6b00748. It is simple to implement
and derives rigorously from Gauss's principle of least constraint.

(cherry picked from commit 75929ee01b7c34a15e3e2ed3e8e0cbff85fe50dd)