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.