rather than adjusting the communication cutoff, we just print out the minimum value needed and error out

i suspect, this communication cutoff adjustment was included into the code before it was possible to separately set it via comm_modify. stopping with an error message printing the needed/current value is cleaner, in keeping with other modules in LAMMPS and much less problematic.

Includes documentation and examples.
NOTE: VV requires very small timesteps under isoenergetic conditions.
Consider using fix_shardlow instead, since this VV support is
primarily for comparison purposes.

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@15604 f3b2605a-c512-4ea7-a41b-209d697bcdaa

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

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@15489 f3b2605a-c512-4ea7-a41b-209d697bcdaa

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)

(cherry picked from commit 433741564d166a5535bebf0e2487db6788d7871e)