The intermediate note list (before it is translated to LilyPond) can now be printed for debugging.
This is making the type signatures in OutputMain rather overgrown, but I'll probably try to add support for generating 'spacer lists' first (see the LilyPond manual section 9.2.2 Creating contexts - the music / arts example). If I tidy up OutputMain first I'll only have to change it again to support spacer lists.
