* src/gdcmCommon.h : add the GDCM_UNKNOWN constant. This constant is to

   * src/gdcmCommon.h : add the GDCM_UNKNOWN constant. This constant is to
     replace the different values "??", "Unkn", "Unknown".
   * src/gdcmDicomDirElement.h, gdcmDictEntry.h, gdcmDictSet.h,
     gdcmDocEntry.cxx, gdcmDocEntrySet.[h|cxx], gdcmDocument.h, gdcmSQItem.cxx,
     gdcmVR.cxx : use the GDCM_UNKNOWN constant.
   * Appears a bug in gdcmDictEntry : IsVRUnknown always returns false... bad
   -- BeNours