Track progress when processing DICOMDIR files.
Track progress when processing DICOMDIR files.

This completes the implementation of progress tracking within
vtkDICOMDirectory (I think).

hendradarwin/vtk-dicom,hendradarwin/vtk-dicom,hendradarwin/vtk-dicom,dgobbi/vtk-dicom,dgobbi/vtk-dicom,dgobbi/vtk-dicom