gromacs-5.1.0 beta patch

Notice that this only works with APIversion>2, that is
with the to-be-released PLUMED 2.2

Fixes a potential problem in the following commits:
423d49bf
bb6178a2
(reported by C. Perego)

Same as 423d49bf applied to gmx 5.0

Needs gromacs to be repatched

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

I added the possibility to pass kbT from gromacs to plumed.
To allow compatibility between old and new versions, I used the API
number. Namely: gromacs checks that plumed API>1; if so, pass the
temperature.

KbT is only used in METAD so far (not in HISTOGRAM)

Addresses #35