It has been a long time since I saw museumgoers as fully engaged as the crowds moving through “Inventing Abstraction: 1910-1925,” the visual and intellectual banquet at the Museum of Modern Art this winter. Visitors are wide-eyed, attentive, quietly exhilarated. And why not? They are having the kind of full-out artistic experience on which the Museum of Modern Art built its legendary reputation, but which it has rarely managed to produce in recent years.