No sejourn in the Washington DC area can be considered complete without a visit to the Wolf Trap Foundation for the Performing Arts, I'd heard on many occasions in the past.The open air, wood-constructed auditorium located some 20 miles outside of the city is the main place to go around here in the summer to experience high profile acts from Wilco to the NSO at a relatively low price.Picnicking on the lawn is the way…