Implement more sophisticated analytic and numerical noise solutions / approximations

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](https://aip.scitation.org/doi/abs/10.1063/1.4927916)
[Forgoston *et al.* (2009) Accurate noise projection for reduced stochastic epidemic models. *Chaos* **19**, 043110](https://aip.scitation.org/doi/abs/10.1063/1.3247350)

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