This method converts a Sparse6 or Graph6 string S to an instance of the class Graph, stored in G.
i1 : stringToGraph "Dhc" o1 = Graph{0 => {1, 4}} 1 => {0, 2} 2 => {1, 3} 3 => {2, 4} 4 => {0, 3} o1 : Graph