Are Flamelet objects generators? #24
Description
We need to clarify the use of state within the flamelet class. For certain things like steady solves we simply use Flamelet instances once and keep rebuilding new ones (build_adiabatic_slfm_library), while in other cases we build a Flamelet and "keep it around" for a while (nonadiabatic, nonstrained library builders). This is a subtle distinction but it's important. We've redesigned around FlameletSpec, which is a persistent container of data required to set up a flamelet calculation. Should we reduce the Flamelet class to functions that return FlameletResult instances that are similarly persistent? It's not ideal to have an ambiguous mix between single-use and persistent data structures, which I think Flamelet falls into right now.
This is relevant to arc length continuation. Do we treat that like transient flamelets where a Flamelet instance provides the entire trajectory, or because we are continuating over the dissipation rate in arc length, do we have to make new Flamelet instances (in which case it wouldn't make sense for Flamelet to have an arc length member function).