Append user-supplied data to chunk, return resulting chunk size. If the
data would exceeded the available space, the chunk grows in size.

def extend(self, data, size):
        """
        Append user-supplied data to chunk, return resulting chunk size. If the
data would exceeded the available space, the chunk grows in size.
        """
        return lib.zchunk_extend(self._as_parameter_, data, size)