Update RaceCarnival, comments and tests

RaceContestant
- updated setCarnivalName to check if name length is > 0 rather than
checking if == "null"
- added comments for startRace() and finishRace()
RaceCarnival
- updated setRaceContestantStartTime and setRaceContestantFinishTime to
include the new TreeMap as well
- added comments in writeContestantFile and readContestantFile