Create a new label

        Arguments:
            doc --- first document on which the label must be added (required
                    for now)

def create_label(self, label, doc=None):
        """
        Create a new label

        Arguments:
            doc --- first document on which the label must be added (required
                    for now)
        """
        label = copy.copy(label)
        assert(label not in self.labels.values())
        self.labels[label.name] = label
        self.label_guesser.load(label.name)
        # TODO(Jflesch): Should train with previous documents
        if doc:
            doc.add_label(label)
            self.upd_doc(doc)
            self.commit()