Fix Stabat Mater test

[jsbean]

This PR fixes the Schubert Stabat Mater LilyPond test. Down to 51 LilyPond test failures.

[codecov-io]

Codecov Report

Merging #126 into latest will increase coverage by 2.43%.
The diff coverage is 96.82%.

@@            Coverage Diff             @@
##           latest     #126      +/-   ##
==========================================
+ Coverage   57.67%   60.11%   +2.43%     
==========================================
  Files         218      219       +1     
  Lines        2800     2911     +111     
==========================================
+ Hits         1615     1750     +135     
+ Misses       1185     1161      -24

Impacted Files	Coverage Δ
Sources/MusicXML/Complex Types/Direction.swift	100% <100%> (ø)	⬆️
Sources/MusicXML/Complex Types/FormattedText.swift	100% <100%> (ø)	⬆️
Sources/MusicXML/Complex Types/Offset.swift	100% <100%> (+100%)	⬆️
...ources/MusicXML/Complex Types/TextDecoration.swift	100% <100%> (+100%)	⬆️
Sources/MusicXML/Complex Types/Dynamic.swift	100% <100%> (ø)
Sources/MusicXML/Complex Types/Dynamics.swift	100% <100%> (+100%)	⬆️
Sources/MusicXML/Complex Types/DirectionType.swift	42.15% <60%> (+0.32%)	⬆️
Sources/MusicXML/Complex Types/Percussion.swift	0% <0%> (ø)	⬆️
Sources/MusicXML/Complex Types/Technique.swift	50.42% <0%> (+0.84%)	⬆️
... and 6 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9ec7650...ab047d9. Read the comment docs.

[jsbean]

@DJBen, @bwetherfield, I need to bail for the evening. I am also a bit stuck on this one. If either of you want to pick this up, that would be great!

This will require getting the Dynamic / Dynamics / DirectionType / Direction pipeline under control. There are some failing tests to get started.

[jsbean]

@DJBen, @bwetherfield, this fix constrains things a little bit more than the spec re: Dynamics.

In the spec, a DirectionType.dynamics should theoretically be able to hold onto a sequence of Dynamics values. I wasn't able to get that to decode properly. We may need to nest this array into a custom type or do some other (de-|re-)nesting procedure in the decode process. This fix only allows you to have a single Dynamics value per DirectionType.

Several LilyPond tests are fixed by this, so I am ok with this being a stop-gap solution. There is a #warning(FIXME:) in the source, and I will create an issue to track this.

[jsbean]

Long live XMLChoiceCodingKey.