CATSKILL – A dentist from Saugerties was charged Wednesday with possessing a sexual performance by a child in Greene County.State police said Trevor H. Keller, 52, has been a practicing dentist at Greene Pediatric Dentistry in Catskill for three years.Keller was arrested following a two-week investigation by state police and their computer crimes unit, assisted by the Greene County District Attorney’s office.At 11 a.m.