Epic: general reactions with stochastic simulations

[jcschaff]

Motivation

currently only mass action like reactions are compatible for stochastic simulations in vcell. While this is ideal and provides the best statistics, users have requested to allow stochastic simulations of non-mass action kinetics as well - possibly with a model warning which can be ignored.

Completion Criteria

Tasks

• [ ] general reactions in math models for Gibson solver
• [ ] general reactions in math models for NFSim solver
• [ ] general reactions in math models for Smoldyn solver
• [ ] #166