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