In last week’s agenda, I led with picks for modern and contemporary music (some reviews to come this week), and I am happy to do so again this week. September seems to be the month for hearing classical music composed in the 20th and 21st centuries, so get out there and listen, Washingtonians. NEW, SOMETIMES STRANGE: >> Modernist composers often write for new and unconventional instruments. Isabel Ettenauer will give a free concert at the…