Updating langevin to work with QM/MM and RATTLE constraints (from @asod)
This merge request obsotes !197 (closed) and !252 (closed) . It is a rebased version of the latter.
It is a reimplementation of Langevin dynamics based on work by Elvar and Asmus (@asod).
Implements Langevin dynamics as described in http://dx.doi.org/10.1016/j.cplett.2006.07.086