Conservation laws for non-constant species & constant species

[dweindl]

I am not sure if the current handling of conservation laws for non-constant species is correct for models that also contain constant species. I didn't check that in detail, but I have the feeling this might currently go wrong, because the total pool is not conserved in case there are constant species. Similar to #2325.

[plakrisenko]

I think one could test that on the Isensee model. It has both