Added conversion factor code gen class and updated evalReactionRates to return
the model conversion factor. This value is used as the alpha in the dgemv mat
multiply.

The conversion factor code gen takes this into account and divides this out
of any local conversion factors each species might have.

Updated evalVolatileStoichiometry to store stoich values if they
are defined by rate or assigment rules.

Updted evalInitConditions to set the rate rule values for stoichiometries
which are defined by rate rules.

Updated ModelDataIRBuilder to generate accessor code so that the
evalConversionFactor can modify the species amount rates.