Also: Music at Longwood, Star Party at Marsh Creek By Denny Dyroff, Staff Writer, The Times A music event with nearly a century of tradition in Chester County highlights what looks to be a fun-filled, but warm and steamy weekend in the Delaware Valley. The Chester County Old Fiddlers’ Picnic (Hibernia County Park, off Route 340, Wagontown, 610-383-2812, http://dsf.chesco.org/ccparks) is scheduled...