MTA officials have an original idea to deal with subway flooding during heavy rain. Rather than run off untreated water into the harbor that surrounds the city, the MTA is thinking of building giant underground reservoirs to store excess water at places like Parsons Blvd. in Queens, which has a spillover effect, so to speak, on the rest of the subway system. Hours after a downpour, huge puddles of rainwater remain near the Parsons Blvd.......