Fixes #132 for gmx 4.6.7

Workaround for #132.

Notice that gromacs wants something like -0.5*f*x,
whereas plumed provides something like -f*x.

Additionally, for not so clear reasons, when using plumed without computing
ENERGY the virial contribution from plumed was completely ignored.

There are two workarounds in this patch:
1. In case plumed does not ask gromacs to compute ENERGY, the plumed contribution
   to virial is stored on a separate tensor and added to the gromacs one at the end
2. In case plumed asks gromacs to compute ENERGY, the a temporary tensor
   is created containing twice the gromacs virial and passed to plumed. The virial
   is then set to half of it.

This should solve both the issues discussed in #132

Needs gromacs to be repatched.