Implement more sophisticated analytic and numerical noise solutions / approximations

[jarmarshall]

Currently basic van Kampen expansion and Stochastic Simulation Algorithm are implemented (showVanKampenExpansion() and SSA() respectively).

Implementation of more sophisticated techniques would enable more complicated systems to be analysed, e.g. time-inhomogeneous systems (although this would require a means of defining schedules for rates).

Some representative references: Thanh and Priami (2015) Simulation of biochemical reactions with time-dependent rates by the rejection-based algorithm. J. Chem. Phys. 143, 05410 Forgoston et al. (2009) Accurate noise projection for reduced stochastic epidemic models. Chaos 19, 043110

In particular being able to analyse systems other than mass-action would be highly desirable.