Major new capability: variable stoichiometries.
Support for variable stoichiometries is now present and should be complete: all the SBML Test Suite cases with variable and named stoichiometries pass, and stoichiometry IDs can now be set and reset, behaving exactly like normal parameters. This even works when multiple species references refer to the same species, meaning they get collapsed in the stoichiometry matrix, but each have variable (or even settable) stoichiometries.
This is a minor version update because the save state files have changed: the new stoichiometry information is saved in multiple places.
Also cleaned up some compiler warnings and the CI.