strip_situation_code() strips the situation code into goalie and skater counts for home and away teams, then (from the event owner's perspective) computes man differential and a strength state classification.
strip_situation_code(play_by_play)data.frame with one row per event (play) and added columns: homeIsEmptyNet, awayIsEmptyNet, homeSkaterCount, awaySkaterCount, isEmptyNetFor, isEmptyNetAgainst, skaterCountFor, skaterCountAgainst, manDifferential, and strengthState
data.frame of play-by-play(s); see gc_play_by_play() and/or wsc_play_by_play() for reference