WIP-MEI: New: Import <space> elements to note.Rest objects

It's not ideal, but it's maybe the best solution.
WIP-MEI: Testing: Unit and integration tests for staffDefFromElement()

Thus entering this function in the realm of the properly tested.
WIP-MEI: New: staffDefFromElement processes <clef> tag inside

With a test, too!
WIP-MEI: Formatting: removed a 'note-to-self' comment
WIP-MEI: Testing: scoreDefFromElement() now has a unit and an integration test
WIP-MEI: Formatting: A host of minor code-style improvements
WIP-MEI: Python 3 called me out...

I was comparing an int with a str! I made some other py3 improvements too.
WIP-MEI: New: <mSpace> imported with mSpaceFromElement()
WIP-MEI: Fixed: New XPath search for initial <scoreDef> elements.

Previously, this XPath query would erroneously pull out <scoreDef> elements held
in the <incip> section. This change appears to have fixed that.
WIP-MEI: New: Additional debugging statements to track convertFromString()

These help me know where errors happen, and which parts take a lot of time.
Hint: it's the slurs.
WIP-MEI: Tests: New test for _timeSigFromAttrs()
WIP-MEI: Test: New tests for _keySigFromAttrs()
fix lilytranslate for svg
WIP-MEI: Tests: Wrote tests for (and corrected) _transpositionFromAttrs()

As it turns out, there was a bug in this function such that the single
non-automated test case I was using (for Clarinet in A) also happened to use
the only possible values that would work.
Merge branch 'master' into mei