The University of Massachusetts says it plans to boost its presence in downtown Boston by opening an academic and administrative building in 2016. All five of the UMass campuses will have the capability of offering classes at the One Beacon Street location. University officials say the classes will be aimed at people who work in the city and want to continue their education. UMass president Robert Caret said the new facility, to be known as the UMass Center on Beacon Hill, represents a major step forward for the school. Caret and other administrators will move there from their current Franklin Street offices. The school has signed a 15-year lease beginning January 2016 for the 31st and 32d floors of the building, which will also house the UMass Club, a meeting place for alumni.