Here is my version of the harmonic. It removes the internal scaling
factors since those only make sense given various assumptions about
the system. Instead offers a function which allows you to compute the
spring constant from a standard deviation (and a temperature) if you
want those assumptions to hold. This way harmonic can be used to
construct more complicated potentials as well as to mimic modeller/
charm if desired. No existing code meaningfully depended on the old
behavior. The test code has been patched.