Implement polymatrix game generators #197

At https://github.com/polymatrix-games/polymatrix-generators there are a number of classes for creating various interesting classes of polymatrix games. It would be useful to incorporate these.

See also #195 on generating bimatrix games. Compared to that issue there is rather more thinking to be done on this one. Gambit does not have a native polymatrix representation as yet. Therefore, one could create polymatrix games, but to use them one would collapse them to a standard N-player game, and lose the polymatrix-ness (is that a word?) and the computational advantage from that.