Add an epsilon_scale() method for games.

When testing, we often need to know how lenient to be when making
comparisons. Trying to figure out just how lenient has been a huge
source of problems, and ultimately, the scaling factor (applied to
ABS_TOL) probably depends on properties of the solution.

This commit adds a new method, epsilon_scale(), that returns a safe
(very lenient) scaling factor. If things are within epsilon_scale()
times ABS_TOL, we consider then equal, or optimal, or whatever.