UNCOVERING the history behind supposedly scandalous nuns has helped fuel interest in archaeology projects in Oxford.     About 500 volunteers from Archeox, the Archaeology of East Oxford Community Project, have been excavating a medieval nunnery at Minchery Farm Paddock, between Blackbird Leys and Littlemore.     The five-week dig finished on Friday when Oxford University Vice Chancellor Prof Andrew Hamilton paid a visit to see some of the historic finds.