Leonard Slatkin seems to have become the go-to conductor for orchestras in a tight scheduling spot. Twice this season alone, he has stepped in to lead the Pittsburgh Symphony when other conductors took ill at the last minute, and next week, he’ll lead the first half of the PSO’s European tour. Most conductors hate being pressed for time, but Slatkin claims to actually enjoy these unexpected engagements, even when they mean canceling his vacation.