* gdcmDocEntrySet::SQDepthLevel and gdcmDocEntrySet::BaseTagKey attributes

  * gdcmDocEntrySet::SQDepthLevel and gdcmDocEntrySet::BaseTagKey attributes
    moved away from gdcmDocEntrySet (since this class is an abstract class
    acting like an interface). SQDepthLevel and BaseTagKey are now
    in class
    - src/gdcmDocEntrySet.[h|cxx] removal of SQDepthLevel and BaseTagKey
      and associated accessors. Doxygenation of the class.
    - src/gdcmSQItem.[h|cxx] SQDepthLevel and BaseTagKey and associated
      accessors added.
    - src/gdcmSeqEntry.[h|cxx]: constructor doesn't handle depth anymore.
      Use SQDepthLevel accessor instead. ::Print() adapted.
    - src/gdcmElementSet.cxx changed according to changes in gdcmSeqEntry.
    - src/gdcmDocument.cxx changed accordingly.