ENH: Implement NE computation by HomotopyContinuation.jl

[oyamad]

A Nash equilibrium of a game with N > 2 players can be represented as a solution to a system of polynomial equations as a nonlinear complementarity problem.

HomotopyContinuation.jl is amazingly powerful to be able to solve for all (isolated) solutions to that system of polynomial equations! (Of course, only for small games).

The function hc_solve just passes the data to HomotopyContinuation.jl, lets it solve the equation, and sorts out nonnegative solutions for Nash equilibria. Then we have all (isolated) Nash equilibria of the game!

This is really amazing. I thought we would need to search through all the possible supports for mixed actions to solve for each profile of supports the polynomial equation of the indifference condition (cf. #47). There is no need of this with HomotopyContinuation.jl: it automatically constructs a start system (t=1) for homotopy continuation, the number of whose solutions is equal to the maximum number of those of the target system (t=0), and follows all the solution paths from t=1 to t=0.

[oyamad]

It will still be interesting to implement the support enumeration algorithm that solves for each support profile the corresponding system of polynomial equations by HomotopyContinuation.jl with only_non_zero option, and compare the function in the current PR.