WIP-MEI: Refactoring: rewrite beamTogether()

I did this a while ago, so I don't remember why... but the diff suggests it's
much cleaner now!
WIP-MEI: Newish: Nested <beam> and <tuplet> Tags Starting to Work
WIP-MEI: Fixed: Tuplets Have Improved Accuracy and Brackets-with-Fractions

Nested tuplets, and tuplets with inconsistent durations, still aren't imported
properly. However, we now have tuplet grouping brackets with fraction numbers
and better detection of durations.
WIP-MEI: Improved handling of <scoreDef> tags outside of <section> tags

Previously, every <scoreDef> would be processed before any of the <section> tags
were processed, then re-processed if they appeared in a <section>. Now, only the
first <scoreDef> tag not in a <section> tag is processed before the first
<section>.
WIP-MEI: Improved handlind of initial Instrument via the <staffDef>

Now it looks clearer what's going on. Plus, I added a TODO for trying other
strategies to get instrument info.
WIP-MEI: New: Process all <staffDef> elements in a <section>
WIP-MEI: Formatting: New "ignored" element and better documentation thereof

The People deserve to know that I'm not (yet) concerned with their thing-breaks.
WIP-MEI: add some minor TODOs that didn't fit elsewhere
WIP-MEI: New: Tuplets given with a <tupletDef> appear to work

Special mention to @WeiToGo for suggesting I use this strategy!
WIP-MEI: Formatting: Add some section-dividing comment-things
WIP-MEI: New: Template for the tupletSpanFromElement() Function

This is for proper <tupletSpan> elements, which have a @plist attribute that
gives all affected objects. Of course, few things in the MEI examples bother
to have proper <tupletSpan> elements, so this isn't urgent.