ZienkiewiczZhuEstimator.
This support is based on the update in ParGridFunction::ComputeFlux
which, in turn, is based on the support for GroupCommunicators in
ParFiniteElementSpaces on non-conforming meshes.
method SetFluxAveraging) that corresponds to the optional
parameter 'with_subdomains' of the function ZZErrorEstimator().
In ex22, add a command line flag to set the new flux-averaging
option in class ZienkiewiczZhuEstimator; also, save the reference
and deformed meshes and displacement field at the end of the
adaptivity loop.