By Frank Kummer The Philadelphia Inquirer Dark water formed an eddy around Steve Eisenhauer's boots as they sank into the muck at the base of a 90-foot black gum tree so old, its roots were deep in this ground when the Pilgrims landed. Scientists estimate the age of black gums in an old-growth forest surrounded by Bear Swamp in Cumberland County, N.J., ranges from 400 to 500 years, making it among