Most grandfathers leave their grandsons a watch or a little cash. Cellist Evan Drachman’s grandfather left him a 1725 Stradivarius. (Of course, Drachman’s grandfather was the legendary Russian cellist Gregor Piatigorsky, who happened to own two Stradivarii.) Drachman, who studied at both the Peabody Conservatory and the New England Conservatory,…