Starter helper functionality

[UnravelSports]

Some tracking data providers do not offer starting XI information. We might need a helper function to determine which players on either team are in the starting line up using the first frame of tracking data.

[Alek050]

In the game.metadata.{side}_players there is a boolian column 'starter'. If this information is not provided, there are two ways to obtain this information:

• event data: check all unique players that performed an event before the first substitute, or if a player is substituted in but has performed an event somewhere during the game, he/she must be a starter.
• tracking_data: check the first 22 players (11 from each team) that have numeric data (non nulls) and assume they are the starting 22.

I think there should be a generic function:

• _add_starter_information that inputs metadata, tracking data (defaults to none) and event data (defaults to none). This function will only be called when no starter information is available in game.metadata.{side}_players and add the starter information. The tracking data information will be used if available, else event data. This needs to be checked and added in the get_game functions (also get_open_game, get_game_from_kloppy).