Support locally variable stoichiometric coefficients

[dallan-keylogic]

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In #1617, a user asked about creating a reaction package with stoichiometric coefficients that depend on local conditions. The application was Fischer-Tropsch synthesis, and the partition of products into pseudocomponents of various chain length depends on temperature. I suggested a workaround in which a Constraint on the control volume is deleted and rewritten, but it may be beneficial to explicitly support this functionality in the future.