British composer Edward Elgar wrote his cello concerto in 1919 — soon after the end of World War I — and it’s suffused with the dark weight of that war. All Things Considered host Melissa Block sat down with 30-year-old American cellist Alisa Weilerstein to listen to Weilerstein’s new recording of the piece, which she made with the Berlin Staatskapelle and conductor Daniel Barenboim.