Polish: Minor refactoring of data handling when processing historical stats.

[MattRWallace]

There is some data cleanup happening in the summarize_historical method of the average_player_summarizer. See if it can be moved to the data cleanup method without regressions in the training stage.

NHLPredictor/src/nhl_predictor/model/summarizers/average_player_summarizer.py

Line 63 in fcdf75e

data[DB.skater_stats_table_name] = data[DB.skater_stats_table_name].set_index(Keys.game_id)