added correction for PME virial with non-linear virtual sites

added correction for PME virial with non-linear virtual sites

With non-linear virtual site constructs the virial nees to be corrected
when it is not calculated from the particle coordinates and masses.
Currently this is only a problem for the PME mesh contribution.
This would lead to an incorrect virial and pressure. The error was small
for unordered systems, but could be significant for e.g. an all-atom
bilayer with hydrogens replaced by virtual sites. Fixes #908

Change-Id: I27a06019aa09734a14f3630ef296aaaacb9c5f0b