WIP-MEI: New: Handling @slur and @tie attributes on <note> and <chord> Elements

To help with this, I moved slur-handling into a separate function that can be
called from anything, to easily add slur-handling to new tags.
WIP-MEI: Formatting: Cleaner Slur/Tie Pre-Processing

They now share an inner function to set attributes.
WIP-MEI: Add some TODOs to mei/__main__.py
WIP-MEI: Tests: Fixed Tests for mei.__main__.makeDuration()

They now work with occasional fractions.Fraction return values.
WIP-MEI: remove unnecessary alias for uuid.uuid4()
WIP-MEI: Tests: Revised _articulationFromAttr() Tests

Previously, the tests were wrong because a mocked return value wasn't
callable but should have been.
WIP-MEI: Fixed: beamFromElement Uses Helper Function to Add Beams

Previously, beamFromElement added beams internally. There's now a helper
function to do this so "adding beams" can be shared between converter
functions.
WIP-MEI: New: <tuplet> Tags Parsed with tupletFromElement()

We still need to deal with <tupletSpan> however. Not sure how yet.