This time of year, with so many concerts on the schedule, it is sometimes hard to separate what is essential from the rest. If we had to pick this week — and we do have to pick, every week — it would be as follows. >> Last week’s stellar concerts from the National Symphony, with Osmo Vänskä and Leonidas Kavakos, were scandalously underattended. If you like good music but were unable to hear the Finnish…