Merge branch 'nathawani/feature-amr-using-flux-norm' into 'main'

SNES ex27: Poisson problem using primal and mixed form for error estimator using flux norm

See merge request petsc/petsc!7674

DMAdaptor: Complete overhaul
- Add CLASSID and type registration
- Add error estimates based upon a mixed problem
- Add DMAdaptorSetType(), DMAdaptorGetType(), DMAdaptorRegister(), DMAdaptorRegisterA

DMAdaptor: Complete overhaul
- Add CLASSID and type registration
- Add error estimates based upon a mixed problem
- Add DMAdaptorSetType(), DMAdaptorGetType(), DMAdaptorRegister(), DMAdaptorRegisterAll(), DMAdaptorRegisterDestroy()
- Add DMAdaptorGetMixedSetupFunction() and DMAdaptorSetMixedSetupFunction()

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