WIP-MEI: Update "test_file.mei" with note about mid-measure clefs not working

For us, unfortunately, it also means ever <clef> inside a <beam> or <tuplet> or
<layer> will not be exported through MusicXML.

Refer to #28 for more information.
WIP-MEI: Correct terminology in some docstrings
WIP-MEI: Correct terminology in _processEmbeddedElements()

Even at the beginning of July, I apparently thought these things were called
"tags." How embarrassing.
WIP-MEI: Modify allPartsPresent() to use a <score> element

Previously, it used an MeiToM21Converter instance. Now it's going to be called
from the upcoming scoreFromElement() function, because the parts might be
different for different <score> elements in the same file. So allPartsPresent()
will now only pick up those @n values that exist in the current <score>.
WIP-MEI: New: staffGrpFromElement() for (guess what) <staffGrp> elements

An important part of this balanced breakfast.
WIP-MEI: "Modernize" existing scoreDefFromElement() tests

Previously, they used a Mock for the Element, which is just ridiculous.
WIP-MEI: scoreDefFromElement() supports a <staffGrp> held within

This is all going to come together soon, with the brand-new sectionFromElement()
and scoreFromElement() functions! Trust me, it'll make sense.
WIP-MEI: Add new difficulty to "test_file.mei" and friends

There are now three <section> elements, one of which is held in another
<section>. These <section> elements are also preceded by <scoreDef> elements
that specify things that music21 must represent inside the <section>, which
poses an additional challenge while importing.
WIP-MEI: WIP on the way to scoreFromElement() and sectionFromElement()

Those functions already exist, and maybe they're fine. They share a helper
function for the actual looping-and-processing bits, which will make them
easier to test. But there are no tests yet. (Of course, there were no tests for
this stuff anyway, since it was all in MeiToM21Converter.run()).
WIP-MEI: remove unused var from measureFromElement()
WIP-MEI: extra newline for easier testing