Merge branch 'merge-pull-153' into lammps-icms

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)