units of compartment area/volume in reaction rates

[lkeegan]

Currently, when a reaction rate equation references the name of a compartment, it uses the numerical value of the volume in units of L^3 or L^2.

In the case of reactions imported from COPASI, typically this factor is symbolically removed on import, but depending on COPASI user choices for membrane reactions it may not be, and it should then be in units of VOLUME, not LENGTH^3, to preserve the behaviour of the model. (thanks @baronvonderheyden)

Todo:

• probably compartments should be in units of volume in reaction rates
• probably membranes are correct as L^2
• confirm the above

[lkeegan]

Also, related to #588 , the size and units attributes of the compartment or membrane in SBML should be consistent with the above.