[IMP-dev] Harmonic cleanup

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.