Modify a player rating map inplace

Algorithms returning by value is a bit ineffecient, especially for large
data sets (RRI?). This patch is the start of converting algorithms to
generally modify a rating map in place.
Pass a SpPlayerRatingMap to SplitPlayersGreedy

Since we updated our player rating algoritm to return a map instead of a
set, this function should take that as an input.
Add skeleton for RRI
Skeleton passes basic test
Stash
Rough sketch of sanity checks for RRI

We'll need to add a DatedGame so we can get season bounds.