Given the name of the source, return the database identifier.

def _id_for_source(self, name):
        """ Given the name of the source, return the database identifier. """
        if not name in self.source2id_lut:
            self.c.execute("INSERT INTO sources (name) VALUES ( ? )", (name,))
            self.source2id_lut[name] = self.c.lastrowid
            self.id2source_lut[self.c.lastrowid] = name
        return self.source2id_lut[name]