Volume data were not exchanged properly in hrex.
Bug was appearing only when not using domain decomposition.

gromacs-5.1.0 beta patch

This reverts commit b148b715.

gromacs-5.1.0 beta patch

it should works, but more tests are needed

Same as 55bc92d2
(written by Carlo), but in such a way that it results as a git mv

The contribution from gromacs (DeltaP*DeltaV) should not
be counted with hrex. Indeed, this term is already included
in the foreign energy calculation.

This now allows to activate hrex from the command line:
mdrun -hrex

As of this commit, to patch with Hamiltonian replica exchange
just use the gromacs-4.6.3 patch

This will simplify keeping up to date with upstream changes in the patch.