For the last 15 years, Normand Mainville, owner of the mid-century modern furniture store Machine Age, has been a Fort Point Channel pioneer, living and working in the artsy industrial loft neighborhood.But three months ago, Mainville moved out of his 1,800-square-foot loft and into a very sleek and sunny 1,440-square-foot condo in the Trolley House, a new building on West Second Street along the industrial – but up-and-coming -- fringe of South Boston.