Concentration to number of molecules converter

[blakeaw]

Suggested by James Pino:

Something like a units.Model.to_molecules(vol=...) converter that can take a model in concentration units and quickly yield its molecular counts. This would be useful for stochastic simulations.

[blakeaw]

This is partially addressed by PR #4, which adds the new SimulationUnits class and the set_molecule_volume function that can be used to enforce automatic conversion of parameters with molar concentrations to molecular counts as part of the model definition.

However, the functionality of something like a Model.to_molecules function has yet to be fully implemented. This is a bit more difficult since the units for expressions that include a concentration must be reconstructed after applying the appropriate conversion to parameters and observables.