Add more docs for a few private methods and implement the do-over.

Some examples and return type documentation were added for the
_zero(), _A(), and _G() methods of SymmetricLinearGame. The _zero()
method was also uncached, since attempting to cache _A() and _G()
revealed that CVXOPT clobbers them. The zero vector appears safe, but
let's play it safe.

This also implements a two-stage retry for the solution of a game. Our
default tolerance is now 1e-6, but when we're trying to solve a game,
we try it with 1e7 first. If we can solve it with the stricter
tolerance, then great. If not, we use the default.