Scrappy, young Web start-ups making apps for iPhones may be the darlings of the tech world, but enterprise software companies remain kings of the innovation economy in Massachusetts. And Parametric Technology Corp., a 27-year-old Needham company, is at the top of that heap. Originally known for pioneering the use of computer-aided design, or CAD, software for engineers, PTC - as it prefers to be known - has recently pushed into new areas of manufacturing.