NMarket|MARKET_WITHERING_LOCATION_THRESHOLD

Withering markets: a market with <= MARKET_WITHERING_LOCATION_THRESHOLD locations that has been at or below the threshold for MARKET_WITHERING_GRACE_MONTHS months AND is outclassed for at least MARKET_WITHERING_OUTCLASSED_FRACTION of its locations gets flagged as withering. AI gets a destroy bias and a player-facing event fires on the on_market_withering_started on_action.