Improved how cupSODA.py handles volumes. If a volume is passed through
the 'vol' argument, the code will now automatically set the
concentration (population) of the '__source()' species to
6.0221413e23*vol. This is necessary because for models defined in number
units cupSODA automatically divides all of the populations by this
factor. This ensures, therefore, that the concentration of '__source()'
equals 1. In general, however, it's advisable not to use this feature.
If a volume is not provided then cupSODA will assume that the model is
in concentration units, even if it is not. Simulations will run fine and
concentrations can be recovered by the user in post-processing.