Add a new switch to the topology.

def addRouter(self, name=None):
        '''
        Add a new switch to the topology.
        '''
        if name is None:
            while True:
                name = 'r' + str(self.__rnum)
                self.__rnum += 1
                if name not in self.__nxgraph:
                    break
        self.__addNode(name, Router)
        return name