Add explanations and references for each algorithm

[kaizu]

• ode

Ordinary Differential Equations. "Explicit Euler", "Cash-Karp", or "Controlled Rosenbrock 4". See Boost.Numeric.Odeint
https://www.boost.org/doc/libs/release/libs/numeric/odeint/

• gillespie

The Gillespie algorithm. The direct method.
https://doi.org/10.1021/j100540a008

• meso

The spatial Gillespie algorithm. The mesoscopic cubic-lattice-based algorithm. The next subvolume method.

• bd

The Reaction Brownian Dynamics (RBD) method.

• spatiocyte

The microscopic HCP-lattice-based algorithm. The Spatiocyte algorithm.

• egfrd, sgfrd

The enhanced Green's Function Reaction Dynamics (eGFRD) method.