Improve Optimal Factorisation Heuristic

[jannikmi]

Optimal Horner Factorisation A* search:
The current heuristic to estimate the minimal number of required operations (lower bound) is to optimistic. This often causes every possible factorisation to be evaluated. For larger polynomials this makes this algorithm infeasible. -> Make the heuristic more accurate.

Attention: Tradeoff between accuracy of heuristic (how many factorisations are being skipped) and the time required to compute the heuristic for every node (is it cheaper to just try a factorisation?)

[jannikmi]

cf. heuristics proposed in

[2] M. Kojima, “Efficient evaluation of polynomials and their partial derivatives in homotopy continuation methods”, Journal of the Operations Research Society of Japan, vol. 51, no. 1, pp. 29–54, 2008.