Missing si_T torque conversion from lbm to si units

[trparry]

I think we are missing si_T, to convert torque from lbm units to si units. Its probably just adding the line
float si_T(const float T) const { return T * kg * sq(m) / sq(s); } //torque si_T = T*[kg*m^2/s^2]
to units.hpp.

[ProjectPhysX]

Yes, thank you! I just added the conversion functions to the repository.