* Preparation of writing a gdcmHeader iterator: generalisation of gdcmTagKey

  * Preparation of writing a gdcmHeader iterator: generalisation of gdcmTagKey
    - The following is the doxygen comment of the typedef declaration
      of gdcmagKey in src/gdcmCommon.h:
           gdcmTagKey is made to old an "universal" (as in URL, Universal
           Ressource Locator)  key to a gdcmDocEntry i.e. a dicom tag.
           A dicom tag allways has a group and an element, but a set of tags
           embeded in various (optionally nested) sequences and sharing
           the same group and element all share the same (group, element)
           "identifier". Hence the (group, element) cannot be used as an
           identifier (in gdcm we shall refer to a "TagKey") of a tag.
           In order to construct a proper tag identifier (i.e. a key) we
           consider the following definition of a TagKey:
           - let Group, Element be the string representation of the
             group and element dicom tag members,
           - let ItemNumber be the string representation of the integer
             index of the considered item number of a sequence,
           Let the key of a tag embeded in a sequence, noted SeqTag, be
           the form:
              /ItemNumber#Group|Element
           where "/", "#" and "|" are characters acting as separators.
           Then the general form of a gdcmTagKey is given by:
              Group|Element<SeqTag>
           where <SeqTag> means NO or many instances of SeqTag.
           Hence the gdcmTagKey of a tag not "leaving" in a sequence is the
           string e.g.
               0028|1201
           but the gdcmTagKey of a tag "embeded" is the first item of
           a sequence, itself nested in the third item of a sequence is the
           string e.g.
               0004|1220/2#0008|0082/0#0008|0090
    - src/gdcmDocEntry.h: added a new Key (of type gdcmTagKey) member, in
      order to hold the new sequence compatible key. Previously, the
      GetKey() method would look in the underlying gdcmDictEntry.
    - src/gdcmDocEntry.cxx:
      -- constructor now copies the underlying DictEntry key, in the local
         Key member.
      -- ::Print: displays the member Key, instead of the (group, element).
    - src/gdcmCommon.h: added some comments on typedef gdcmTagKey.
    - src/gdcmDocEntrySet.h:xi
      -- ::ParseDES() now setups the gdcmTagKey of the sequence it is parsing.
      -- now has a new BaseTagKey member.
      -- STYLE.
  * src/gdcmValEntry.[h|cxx], src/gdcmBinEntry.[h|cxx]: the member VoidArea,
    previously a member of gdcmValEntry, moved to gdcmBinEntry were is
    truly belongs.
    This poses the problem with the semantics of the following lines
       LoadEntryVoidArea(0x0028,0x1201);  // R    LUT
       LoadEntryVoidArea(0x0028,0x1202);  // G    LUT
       LoadEntryVoidArea(0x0028,0x1203);  // B    LUT
    in gdcmDocument::gdcmDocument(std::string const & ). Please refer
    to the long FIXME note for what the problem is. Nevertheless in
    order to get things working the dicom dictionary was altered !
    Please fix things urgently...
  * Dicts/dicomV3.dic WRONGLY altered (this means we introduced a uncorrect
    information), see above note on moving the member VoidArea. Nevertheless
    the following entries previously correctly set as US are now inproperly
    set to OW:
      0028 1201 OW IMG Red Palette Color Lookup Table Data
      0028 1202 OW IMG Green Palette Color Lookup Table Data
      0028 1203 OW IMG Blue Palette Color Lookup Table Data
  * src/gdcmDocEntry.[h|cxx], src/gdcmSeqEntry.h: SQDepthLevel member
    of gdcmDocEntry moved to gdcmSeqEntry.
  * src/gdcmSeqEntry.cxx: STYLE.