* src/gdcm.h, src/gdcmHeader.cxx: added GetPubTagNames() resp.

  * src/gdcm.h, src/gdcmHeader.cxx: added GetPubTagNames() resp.
        GetPubTagNamesByCategory() to gdcmHeader that return a list of the
        entries within the associated public Dicom dictionary resp. the same
        information but sorted by the fourth field (PAT, IMG, DIR) of the
        dictionary.
        - Dicts/dicomV3.dic Entries which had and unspecified fourth field
          are now in the "???" group.
        - python/gdcm.i changed accordingly,
        - python/demo/printGroupedPublicDict.py added, that gives an example
          of the above new functionalities in Python.  --- Frog