PETSc DAE Index Reduction Fix

[dallan-keylogic]

Summary/Motivation:

This PR is intended to allow PETSc to be used in problems where the DAE index has been reduced by deactivating some discretization equations and adding additional constraints. Frequently in IDAES models, we end up having to deactivate the discretization equations for a specific index, e.g., m.fs.unit.control_volume.material_accumulation_disc_eq[:, "Liq", "H2O"].deactivate(). This PR adds checks about whether the discretization equations for that index are active, then adds a DerivativeVar to the set of initial conditions to be fixed only if it is active.

It also contains a workaround for Pyomo/pyomo#3734 .

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[dallan-keylogic]

@blnicho , @Robbybp , @eslickj , this fix causes issues with a few tests when a central or forward difference scheme is used. What should we have when the user discretizes with a Forward or Central scheme and no discretization equation exists at the final time?

[ksbeattie]

@blnicho says she should have time to look at this by early next week.