Rewrite thermal drag, gamma, calculation to be more sensible. No substantive changes.
Rewrite athermal drag, gamma_ath, to be independent of thermal term, and scaled correctly. But still uses non-deterministic force
Add code to track conservative bit of the force, callled f_det confusingly (even though still doesn't include propulsive bit).
Change omega thermostatting to use conservative force rather than conservative+langevin force (langevin pre rotdiff of course)
Merge branch 'collisional_rotdiff' of https://github.com/eddiejessup/lammps into collisional_rotdiff

Conflicts:
	src/fix_langevin.cpp