Add <=> so call numbers are sortable.  One test fails, I don't know why

Bill---

This is probably quite badly done, but it's a start. It adds a <=> to the CallNumber class so that call numbers are sortable. I made a few simple tests, but a) there must be a better way to do them, and b) one fails and I can't see why. Maybe you can see why? I hope there's something useful here.

(Looking at the diffs it looks like a lot of whitespace changs because Emacs chomps trailing spaces. Sorry if that's annoying.)

Bill