add updated ranks and versions

[aspriddell]

This brings a few changes (nothing too breaking though):

• IsLegacySeason has been removed from SeasonStats because it'll have no meaning (as we've now got 3 rank generations)
• RankInfo is now a struct to improve allocation performance (still a micro-optimisation but whatever)
• Extra XMLDoc for the SeasonalRanks class
• Modified tests to better test invalid ranks (and work with the changes)