Move the page number by a given offset. Beware to not let any hole
        in the page numbers when doing this. Make sure also that the wanted
        number is available.
        Will also change the page number of the current object.

def change_index(self, offset=0):
        """
        Move the page number by a given offset. Beware to not let any hole
        in the page numbers when doing this. Make sure also that the wanted
        number is available.
        Will also change the page number of the current object.
        """
        src = {}
        src["box"] = self.__get_box_path()
        src["img"] = self.__get_img_path()
        src["thumb"] = self._get_thumb_path()

        page_nb = self.page_nb

        page_nb += offset

        logger.info("--> Moving page %d (+%d) to index %d"
                    % (self.page_nb, offset, page_nb))

        self.page_nb = page_nb

        dst = {}
        dst["box"] = self.__get_box_path()
        dst["img"] = self.__get_img_path()
        dst["thumb"] = self._get_thumb_path()

        for key in src.keys():
            if self.fs.exists(src[key]):
                if self.fs.exists(dst[key]):
                    logger.error("Error: file already exists: %s" % dst[key])
                    assert(0)
                self.fs.rename(src[key], dst[key])