made PME work with a mix of 1 and more threads

Using a mix of 1 and more OpenMP threads on different MPI ranks
would make mdrun terminate with an MPI error.
Fixes #1171

Change-Id: Iffa16e18baf0f74be826b59503208dca01d1ec14
Merge "made PME work with a mix of 1 and more threads" into release-4-6