value in WDL

a decisive result according to WDL

* When eval is close to zero, this allows users to more easily identify
  which "drawn" positions are actually dead drawn vs.
  unclear/complex/sharp
* When eval favors one side, this allows users to more easily identify
  which portions of the game contained more counterplay/complexity
  despite the advantage

Help to reduce merge conflicts w/ rooklift#237

Review response to rooklift#248 (comment)

rooklift#248 (comment)

Draw as a "WDL graph" instead

* Handle null values correctly

Directly use the `WDL_mu` stddev formula `2 / (ln(1/W-1) + ln(1/L-1))` to calculate the interval around the eval line.

"I just realized that optically it suggests the opposite of what your initial plots did (i.e. the draw area going to 100% in case of a draw), and if you stick to the idea of the shaded areas representing uncertainty about the eval, I think the currently black part should be shaded instead"

"There are probably some edge case issues with `W` or `L` being reported as 0 you need to deal with, e.g. by scaling WDL by 0.994 and adding 0.2% to each of W,D,L when calculating the line."

(and leaving it to the user to use adequate WDL sharpness through the contempt settings)