these tests fail with it the other way.
C_CTestXdmfWriter (Failed)
C_CTestXdmfAggregate (Failed)
Cxx_TestXdmfAggregate (Failed)
Cxx_TestXdmfFunction (Failed) new fail
Cxx_TestXdmfSubset

xml file for aggregate is no different, but reading
balks at lack of "Content" the other way around