A large-scale training exercise will bring first responders together in Houlton on Sunday, May 17. The mass casualty simulation will take place from 7:30 a.m. to 6 p.m. at the Houlton Fairgrounds, Houlton Ambulance Service officials said. This is the first time in several years that Houlton has hosted such an event, staff said Monday. The event is solely a training exercise and there is no real emergency, but people will hear loud noises and sho…