Add units to model parameters, expressions, species

[ortega2247]

Hi, mistakes in units can lead to unexpected simulation results and some times hard to interpret results. Dimensional errors happen quite often. Would it be too hard to add dimensional analysis to pysb? I have been thinking we could add a unit argument (sympy has units) to parameter, expression objects, species and evaluate if the reaction rates have the desired units. Is something like this feasible?

[lh64]

I think this is a good idea. Could we add a 'units' attribute to the 'Parameter' class? If we did, it should be possible to write a script that confirms that all units in a model are consistent. I'm not sure if we'd need to add a 'units' attribute to the 'Expression' class as well, or if units could be automatically derived for expressions based on parameter units. Something to think about, though.

[FFroehlich]

Agree that this would be interesting.

I think it should be possible to infer units for all model components from Monomer, time and parameter units for most models. Units for expressions could be necessary for some models, but that could also always be addressed by converting some factor in the expression to a parameter. Yet, it would be nice to make specification of units for expressions, observables, etc optional and only check consistency when units can be inferred from constituent symbols.