Fixes for relative thermostatting.

The Drude thermostat now works, at least in the context of tau_t
being equivalent between atom and Drude tc-grps.  Need to test
longer simulations with dissimilar tau_t values, but at present,
NVT simulations appear to work correctly.

Change-Id: I043d5965eca56917324db6b0119d3edd40fd8748