Add a label on a document.

        Arguments:
            label --- The new label (see labels.Label)
            doc --- The first document on which this label has been added

def add_label(self, doc, label, update_index=True):
        """
        Add a label on a document.

        Arguments:
            label --- The new label (see labels.Label)
            doc --- The first document on which this label has been added
        """
        label = copy.copy(label)
        assert(label in self.labels.values())
        doc.add_label(label)
        if update_index:
            self.upd_doc(doc)
            self.commit()