“Jannis Kounellis in Six Acts,” the first retrospective of the Greek artist’s work in North America since 1986, opened this past month at the Walker Art Center in Minneapolis. On the occasion of this rare survey, Artforum contributing editor David Rimanelli zeroes in on the Arte Povera giant’s career-spanning “blocked door” series, reflecting on the seemingly contradictory qualities that propel and complicate the artist’s oeuvre.THE LAMEST JOKE about Arte Povera is calling it ricca. I searched my memory and the internet for some hidebound-but-amusingly-so British art critic who made the crack