Generalize dissipation rate specification

[michael-a-hansen]

Currently we're pretty stuck on Peters or constant dissipation. While a user can specify their own dissipation rate, it is up to them to parameterize it. Seems like we should invert some of our design so that "Peters" is just another function for the dissipation rate, which Spitfire manages in general. Ideally someone could provide a function for the dissipation rate and let Spitfire scale it accordingly for a desired stoichiometric or maximum dissipation rate.

We could also consider supporting a general function, say chi(z, user_param1, user_param2, ...) for which we defer evaluation until the additional parameters are provided through a tabulation interface.