toString(MixedGraph) -- print a mixed graph as a string

i1 : G = mixedGraph(graph{{3,1}},digraph {{1,2},{2,3}},bigraph {{3,4},{2,4}})

o1 = MixedGraph{Bigraph => Bigraph{2 => {4}   }}
                                   3 => {4}
                                   4 => {3, 2}
                Digraph => Digraph{1 => {2}}
                                   2 => {3}
                                   3 => {}
                Graph => Graph{1 => {3}}
                               3 => {1}

o1 : MixedGraph

i2 : toString G

o2 = new HashTable from {Bigraph => bigraph ({3, 4, 2}, {{4, 3}, {4, 2}}),
     Digraph => digraph ({1, 2, 3}, {{1, 2}, {2, 3}}), Graph => graph ({3,
     1}, {{1, 3}})}