Added support for generating Drude velocities.

Thermalized Drudes use a separate, low-temperature thermostat.
Support for that thermostat is currently being implemented, but
this commit sets up the ability to generate velocities for Drudes
(shells) according to new .mdp option drude-t.

Change-Id: I8ee32a891dccacbddfa09b6085f98f150cc9d6c3