Who cut the cheese? New Jersey, apparently. Across the length and breadth of Manhattan, people were asking, "What's that smell?" after a pungent odor like natural gas or rotten eggs blanketed the borough and northern New Jersey for three hours yesterday morning. By evening, the answer seemed to be a stinky gas emitted by a New Jersey swamp or marsh. "That's where our noses and instruments tell us" the smell was coming from, said Charles Sturcken, a spokesman for the city Department of Environmental Protection.