Improve Chromatic Polynomial Computation

[DavePearce]

Notes from Gary:

if vertices x and y have n edges joining them. their tutte polynomial is

T(G with n (x,y) edges, x,y) = G with one (x.y) edge, x, y) + (y^{n-1} + y^{n - 2} + ... + y^{2} + y^{1}) T(G with (x,y) contracted and multiple (x,y) edges deleted,x,y)

the polynomial can be stacked with the graph as a value n-i and then the final answer is multiplied by all these similar polynomials collected in the computation of the graph. we do this final multiplication process now but do not make sure there are no multiple edges to be considered in any delete/contract step

[DavePearce]