WIP-MEI: Corrections to "test_file.mei" and friends

Previously, I had an invalid clef specification, and forgot to put it in the
LilyPond file. Now, I corrected the clef spec, and put it in the LilyPond file.
WIP-MEI: MeiToM21Converter.run(): Things in <staffDef> are put in next Measure

Previously, things that came from a <staffDef> between <measure> tags were put
into the Part between Measure objects. The MusicXML export doesn't like this, so
that's my hint that things from those <staffDef>s should be put into the next
Measure instead.
WIP-MEI: More helpful warning message for ignored sub-elements.

In _processEmbeddedElements(), and other functions that handle sub-element
processing, there is now a consistent and more helpful debugging message.
WIP-MEI: Switch layerFromElement() to using _processEmbeddedElements()

For some reason, lFE had a near-exact copy of _pEE() in it. So now it just uses
the separate function itself.
WIP-MEI: Improve staffDefFromElement() and its tests

Tests now use an Element directly, and don't test the calling order of
elem.get(), which was both unnecessary and complicated. Also, the function now
only returns an Instrument when it gets info for one. (It used to always return
an Instrument, leading to lots of uselessly-empty Instrument objects).
WIP-MEI: measureFromElement() imports <staffDef> elements contained within

This is an important step in my recent quest toward better handling of
<staffDef> and <scoreDef> elements generally.