#42 framework types and reaction directions

[Allyson Lister]

It seems as if you're conflating multiple things when you have terms that stand for both a framework type (continuous, discrete) and a reaction direction (forward, reverse). Especially since you put them all in for the forward/reverse case via forward rate constant and reverse rate constant, but don't put them as children in the continuous/discrete case. I can understand that you want a single term that is a child of both a framework type and a reaction direction, but then you have top-level terms for forward and reverse rate constant, but no equivalent top-level terms for the continuous/discrete framework. It would seem a nice symmetry to have these classes.