Check for invalid values added to items.
Check for invalid values added to items.

If an invalid value is added to a vtkDICOMItem, then it is not
added, and any previous item with the same tag is removed.

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