def insert(self, idx, w, comment=''):
        '''insert a waypoint'''
        if idx >= self.count():
            self.add(w, comment)
            return
        if idx < 0:
            return
        w = copy.copy(w)
        if comment:
            w.comment = comment
        w.seq = idx
        self.wpoints.insert(idx, w)
        self.last_change = time.time()
        self.reindex()
insert a waypoint