mjo-common: prevent \Sn[1] and \Hn[1] from dropping their superscripts.

This was a TODO item for a while; a standalone "S" or "H" doesn't have
the same meaning as, say, a standalone \mathbb{R} does. The "n"
doesn't indicate the arity of a Cartesian product, so for example S^2
isn't two copies of S^1. Dropping the superscript was therefore
misleading (and nonstandard notation).