‘Hokusai,” at the Museum of Fine Arts, Boston, is an absolute dream. Almost immediately—in this phenomenal retrospective of more than 230 works by Japan’s most famous artist—it is easy to see why Katsushika Hokusai (1760-1849) continues to be universally revered. Hokusai was, in turns, a romanticist, a classicist and an expressionist; a reverent traditionalist and a pioneering, crowd-pleasing populist.