A man who state troopers said portrayed himself as a Pittsburgh police officer on a Hempfield doorstep Friday morning is behind bars. State police said they were called to a home on Crows Nest Road just before 10 a.m. by a resident who reported a suspicious person knocking on her door. Two men there told a state police trooper they were from out of town and were seeing if anyone wanted gravel for their driveway, according to court papers…