Decoding Fixes and API Refinement

@DJBen and @bwetherfield made great strides in improving the decoding process (e.g., #201, #203, #204, #206) and standardizing the API (e.g., #207, #208).

Under the hood, we added all of the MusicXML sample files to the test suite, and made the testing infrastructure more flexible.

Resolve XMLCoder version

0.2.2

Use tagged version of XMLCoder (#191)

Streamline Score structure

• Score is now the first-class citizen of the MusicXML package
• Header elements have been fixed
• Generalized testing infrastructure has been added (thanks @DJBen!)

Decoding the LilyPond test suite

Pre-release version 0.2.0 now successfully parses the LilyPond test suite in its entirety, with the exception of two tests:

• Compressed-MusicXML, which is not yet critical to support yet
• PartNoId, which is invalid as per the MusicXML specification.

Thanks to @DJBen and @bwetherfield for making this possible 🎉 !

Hello, World!

Provide a model and API for deserializing only the "Hello World" example provided by MakeMusic.