Add ability to compute adjusted WPA

[AndrewRook]

If you take a given play and then look at plays that happened at a similar game state, you should be able to construct a distribution of WPA from that game state (you could imagine this like a markov process, or just binning up nearest neighbors and averaging). Once you have that distribution, you could first compute the WPA, then convert that into a Z-score. Overall this is probably not a huge change, but there could be some issues with unusual distributions in situations near the end of the game. Not sure if it will make a difference or not, but it's something I can keep an eye on.