generally don't insist on numeric clues

There's a couple of places where we expect numbers, e.g. as clues for Skyscrapers. For some other types, we're less strict, e.g. for Coral, because at some point I wanted crypted clues. Probably we should just accept arbitrary characters anywhere.

(That's a bit at odds with the idea of having a file with type "skyscrapers" describing a classic skyscrapers puzzle.)