Hatboro police have the dragnet out for an armed bandit.  Investigators say the lone gunman robbed the T-D Bank branch on South York Road yesterday.   He brandished a pistol as he demanded money from a teller.   The bank robber escaped in a Chevy Blazer.   It was found in Upper Moreland not long after the holdup.   Police say it was stolen from a 7-11 store parking lot in Horsham earlier in the day.