WIP-MEI: Add support for @right="rptboth" in <measure>

Though I made a tested change to _barlineFromAttr(), most of the hard work
happens in measureFromElement() and MeiToM21Converter.run(), which aren't
yet tested. It's also not a very clean solution.
WIP-MEI: Tests for instrDefFromElement() added
WIP-MEI: Improve test for beamFromElement()

After improving the doctest, it's now a good integration test. Since this
function doesn't have any logic, I don't think it's necessary to write
unit tests.
WIP-MEI: Tests added for articFromElement()

Two doctests. This is a very straight-forward function.
WIP-MEI: Tests added for accidFromElement()

Two doctests. This is a very straight-forward function.
WIP-MEI: Tests added for @m21Beam in noteFromElement()

I think the implementation is fine, so I added some tests.
WIP-MEI: Tests added for @m21Beam in chordFromElement()

I think the implementation is fine, so I added some tests.