Register a new entity for this document DTD external subset.

def addDtdEntity(self, name, type, ExternalID, SystemID, content):
        """Register a new entity for this document DTD external subset. """
        ret = libxml2mod.xmlAddDtdEntity(self._o, name, type, ExternalID, SystemID, content)
        if ret is None:raise treeError('xmlAddDtdEntity() failed')
        __tmp = xmlEntity(_obj=ret)
        return __tmp