Make units of force constant configurable

[tgmaxson]

What are the units of the force constant? From reading the src I cannot tell if this should be in kcal/mol or kT units etc.

It would be nice if we could supply the value of kB on the CLI such that we can use whatever unit we would like (or have a table to lookup from such as https://en.wikipedia.org/wiki/Boltzmann_constant#Value_in_different_units)