methods for score at given time

New methods in Game class for getting score at a given time by adding up previous scoring plays.

Methods seem to function accurately (for instance, when tested by looking at summed score-at-end-of-game vs. reported score-at-end-of-game), however there do seem to be a significant number of missing extra points, which there may be ways to mitigate.

Still to come: adding methods to Drive and/or Play to get score before or after (plan is to make before/after an option with default of before=False).