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.

This includes docs, added testing and inclusion of USER-OMP support.

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.