normalize_coordinates() normalizes the x and y coordinates for all the events (plays)
in a play-by-play such that they all attack towards +x. If homeTeamDefendingSide
is not available, the home defending side in period 1 is inferred using zoneCode,
isHome, and xCoord.
normalize_coordinates(play_by_play)data.frame with one row per event (play) and added columns xCoordNorm and yCoordNorm
data.frame of play-by-play(s); see gc_play_by_play() and/or wsc_play_by_play() for reference