Design notes:
  * STL hash table ("#include <map>") requirements:
    1/ iterator: first(), ++, (i.e. iterate or next).
    2/ should be sortable (i.e. sorted by TagKey). This condition shall be
       droped since the Win32/VC++6.0 implementation doesn't look like
       providing this facility.
    3/ has a setup some default size value (in number of items), which should
       be around 4500 entries which appears to be the average dictionary size
       (said JPR).


Examples:
 * gdcmFile WriteDicom;
   WriteDicom.SetFileName("MyDicomFile.dcm");
   string * AllTags = gdcmHeader.GetDcmTagNames();
   WriteDicom.SetDcmTag(AllTags[5], "253");
   WriteDicom.SetDcmTag("Patient Name", "bozo");
   WriteDicom.SetDcmTag("Patient Name", "bozo");
   WriteDicom.SetImageData(Image);
   WriteDicom.Write();


   Anonymize(ostream& output) {
      a = gdcmFile("toto1");
      a.SetPubValueByName("Patient Name", "");
      a.SetPubValueByName("Date", "");
      a.SetPubValueByName("Study Date", "");
      a.write(output);
   }