Rabbit Hole is the most produced play this year nationally — does this mean America likes its theater desperately, achingly sad? Much more likely, artistic directors appreciate the general brilliance of this Pulitzer Prize-winning work, which has been adeptly staged by Olney Theater. It’s a production with a clean, contemporary look (notice the silvery, twisting trees adorning the streamlined house set) and a team of actors with the grace to carry the rapid, conversational…