Add a utility to check whether a file is DICOM.
If SeriesInstanceUID is missing, image is not part of a series.
Add a vtkDICOMDirectory class for DICOMDIR handling.

This class will look for a DICOMDIR first, but if no DICOMDIR is found
then it will search the directory file DICOM files.
Change the sorting order for DICOM directory listings.

Sort patients by name, rather than ID.
Sort studies by date, rather than UID.
Sort series by number, rather than UID.