Add a new switch to the topology.

def addSwitch(self, name=None):
        '''
        Add a new switch to the topology.
        '''
        if name is None:
            while True:
                name = 's' + str(self.__snum)
                self.__snum += 1
                if name not in self.__nxgraph:
                    break
        self.__addNode(name, Switch)
        return name