describe the graph

    :returns: report
    :rtype: string

def get_report(self):
    """ describe the graph

    :returns: report
    :rtype: string
    """
    ostr = ''
    ostr += "Nodes: "+str(len(self.__nodes.keys()))+"\n"
    ostr += "Edges: "+str(len(self.__edges.keys()))+"\n"
    return ostr