A user can currently get a sub-matrix of a MatNest and modify it without
it updating the MatNest's state. This may require a manual call to
PetscObjectStateIncrease so that the PC knows that it needs to rebuild,
but this is ugly/complicated/error-prone. Some possible options off the
top of my head:
1. Insist that the user call MatNestSetSubMats() after any modification.
2. Make MatNestRestoreSubMats and MatNest{Get,Restore}SubMatsRead so
that mutable access is controlled.
3. Add a hook to PetscObjectStateGet that can traverse child objects in
case its own state counter is stale. This is the most automatic, but
I'm concerned about its potential complexity.
