Add normalized game pair Elo to stats

[vondele]

We're observing some nice results with the normalized game pair Elo (ngpElo) which seemingly is a good way to derive an Elo number that is largely book independent, working with game pairs. See some data (and the formula) here:

https://github.com/official-stockfish/Stockfish/wiki/Useful-data#equivalent-time-odds-and-normalized-game-pair-elo

Would be nice to put it in the stats, and possibly even see if it makes sense to replace nElo with it in fishtest, even though that's a larger undertaking.

The ngpElo concept is from @Naphthalin he might be able to explain the properties a bit better.