* gdcm/Doc many doxygen changes:

  * gdcm/Doc many doxygen changes:
        - Doxyfile now has two different versions, DoxyfileUsers that generates
          the html.users end users oriented documentation and
          DoxyfileDeveloppers that generates the html.developper developper
          oriented documentation.
        - README changed accordingly
        - requirement.txt removed (old design info moved to TODO).
        - Additional files DoxyInstallation.txt, DoxyIntroduction.txt,
          DoxyMainPage.txt, DoxyPython.txt provide more information.
        - Note: the website http://www.creatis.insa-lyon.fr/Public/Gdcm/ can now
          be updated by simply regenerating the docs with doxygen and copying
          html.developper and html.users to tux:/home/httpd/html/Public/Gdcm
      * src/gdcmElValSet.cxx: stl <map>.count() can only return 0 or 1.
          Hence all the tests in the form "if (<map>.count() >1)" were removed.
      * src/gdcmFile.cxx: cosmetic changes to avoid messages of doxygen about
        ill-formed documentation sections.
      * src/gdcmHeader.cxx: ditto   --- Frog