Add SerieHelper::AddGdcmFile(File *header) method.

Add SerieHelper::AddGdcmFile(File *header) method.
 * \brief add a gdcm::File to the first (and supposed to be unique) list
 *        of the gdcm::SerieHelper.
 * \warning : this method should be used by aware users only!
 *            User is supposed to know the files he want to deal with
 *           and consider them they belong to the same Serie
 *           (even if their Serie UID is different)
 *           user will probabely OrderFileList() this list (actually, ordering
 *           user choosen gdm::File is the sole interest of this method)
 *           Moreover, using vtkGdcmReader::SetCoherentFileList() will avoid
 *           vtkGdcmReader parsing twice the same files.
 *           *no* coherence check is performed, but those specified
 *           by SerieHelper::AddRestriction()