Allows for KbT to be passed from MD code and used in METAD

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