The Isabella Stewart Gardner Museum’s expansion, designed by the famed architect Renzo Piano, was many years in the making. Created after museum leaders recognized a need to relieve pressures on the century-old palace, the $114 million new wing is a sleek contemporary structure with a striking new concert hall, an expanded cafe and gift shop, flexible gallery space for temporary shows, and improved facilities for conservation, education, and artists in residence. Great expanses of glass invite the eye to look inside.