Propensities to run_results

[szb37]

It would be useful to add more dataframes to the run_results. Specifically, it would be useful to record here:

• the propensities of reactions (after the reaction closest to the checkpoint has fired)
• the abundance of species in concentration space, not just copy numbers

These would be enormously useful for me now when trying to calibrate the models.

I know that the information above could be obtained from the logs, but since log files record information on a per event basis, they can be prohibitively large.

Probably in the long run, the the user should be given some control in what variables are saved in the pickle files.

[artgoldberg]

the abundance of species in concentration space will be available in my next push.

[artgoldberg]

"species concentrations" is done.

[artgoldberg]

"propensities of reactions" added to RunResults as rate_laws. Concentrations added earlier as RunResults.get_concentrations.