Grob::vertical_less and DOWN/UP

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Hey all,

Currently, I've written the function Grob::vertical_less such that it returns 
true for a grob that is higher than another grob.  This is because the ordering 
of a vertical alignment's element list goes from top to bottom, so the vertical 
axis groups with lower indices in the list are higher on the page.  I'm using 
this as well in my footnote patch.

This is theoretically no big deal, as it corresponds to our intuition about how 
scores are organized (if someone asked me "What's the first instrument in an 
orchestral score?" my instinctive response would be flute, not contrabass).  
However, it goes against the way that DOWN/UP works in LilyPond, where down is 
"less" than up.  Thus, Grob::vertical_less yields a result that may be 
confusing for some.

I think that it'd be good to scrub this inconsistency from the code base such 
that "lower on the page" always means "numerically lower" (so the contrabass, 
for example, would be the 0th vertical axis group in an elements list).  
Thoughts?

Cheers,
MS