In the quest to describe the music of Fred Hersch in a word — a preposterous task, but not a pointless one — you could do a lot worse than “refinement.” Mr. Hersch is a pianist of cultivated taste and erudition; he’s also the sort of jazz musician who brings a lissome elegance to his playing, disinclined to accentuate the effort behind it all. But there’s another definition of refinement that has to do with painstaking progress, the incremental stretch toward an elusive ideal.