dd121 regressiontest fails with >=4 MPI ranks + OpenMP - Redmine #1981

E.g. with 4 MPI and 2 OpenMP I get:
checkpot.out (3 errors), checkforce.out (2 errors)
Angle step 20: 31.0352, step 20: 31.5902
Proper Dih. step 20: 20.253, step 20: 19.9199
Improper Dih. step 20: 5.84165, step 20: 5.98111

I get the error both the GCC (4.9.3) and ICC (16).

(from redmine: issue id 1981, created on 2016-05-31 by rolandschulz, closed on 2016-06-27)

• Changesets:
  • Revision 0e31b39c by Berk Hess on 2016-06-26T18:14:27Z:

Fix vsite bug with MPI+OpenMP

The recent commit b7e4f30d caused non-local virtual sites not be
treated when using OpenMP. This means their coordinates lagged one
step behind and their forces are not spread to the atoms, leading
to small errors in the forces. Note that non-local virtual sites are
only used when local virtual sites use them as a constructing atom;
the most common case is a C/N in a CH3/NH3 group with vsite H's.
Also added a check on the vsite count for debug builds.

Fixes #1981.

Change-Id: Ibe13b75b8ae9841937ad4abc007dba5ad78a30cd