WIP-MEI: New: Some metadata fields appear to be imported correctly sometimes.

I'm really not sure how I'll end up modularizing this, so it's very informal.
WIP-MEI: New: Function to handle the "converting inner elements" task

Every element that may hold elements will have to convert them. This function
consolidates that task. I may need to adjust before it's suitably abstracted,
but this is a first try!
WIP-MEI: Fixed: tupletFromElement() uses the new _processEmbeddedElements()
WIP-MEI: Fixed: beamFromElement() uses the new _processEmbeddedElements()
WIP-MEI: Fixed: staffDefFromElement() uses new _processEmbeddedFromElements()
WIP-MEI: Refactor: Use iterfind() rather than findall(), where appropriate

Element.findall() and .iterfind() do almost the same thing, but the latter
returns an iterable rather than a list. This may make a big difference when
importing very large files, so I'm switching to iterfind() whenever the result
is being used for iteration.
WIP-MEI: Tests: I forgot to commit the TestEmbeddedElements test suite
WIP-MEI: Formatting: Change unnecessary use of findall() to find()

I would only ever have used the first element returned by findall(), so why not
just use find()? Well now I am.
WIP-MEI: Tests: Corrected a doctest for Python 3 compatibility