(backported from v2.5)

it includes an additional modification to keep multisim in sync when using plumed

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.