v0.4.0

This release contains breaking changes (but the impact of the breaking changes is small and most users are probably unaffected).

Added

• Ability to save and load the player ratings from a Tracker object.
• Added the option to disable tracking the full history of each player in the Tracker object.
  Intended for use with large datasets, where disabling full history could reduce memory footprint substantially.

Changed

• [Breaking change] Refactored the Tracker object to store players in a list rather than a dataframe.
  The 'player_df' attribute was renamed to 'players' and its type was changed to a list.
  Your code is affected if you previously provided the player_df parameter when instantiating a Tracker object (i.e., Tracker(player_df=...) -- the new syntax is Tracker(players=...)) or if you directly accessed the Tracker.player_df attribute (new attribute is Tracker.players).
• [Breaking change] Removed the logger attribute from the MultiElo, Player, and Tracker objects.
  Technically a breaking change, but it is unlikely anyone was supplying a logger when instantiating these objects.