Returns the chunk object for the supplied identifier
        @type chunk_id: string
        @param chunk_id: chunk identifier

def get_chunk(self, chunk_id):
        """
        Returns the chunk object for the supplied identifier
        @type chunk_id: string
        @param chunk_id: chunk identifier
        """
        if chunk_id in self.idx:
            return Cchunk(self.idx[chunk_id], self.type)
        else:
            return None