Add support for composite datasets to vtkExtractTemporalFieldData.
vtkExtractTemporalFieldData poorly handled composite datasets, esp. those generated by readers like vtkExodusIIReader. Added ability to the filter to use only the 1st non-empty leaf node's field-data to produce the output -- a more suitable solution for vtkExodusIIReader's output, since vtkExodusIIReader duplicates global data on all leaf nodes.
Added a test, extractTemporalFieldData, that tests this filter along with the new functionality.
Needed to address paraview/paraview#16100 (closed).