Use classes instead of dictionaries to represent songs, measures, notes, headers, etc.

[vivaria]

This would:

• Make initializing default notes/branches/measures cleaner
• Make the objects a little easier to inspect
• Make accessing object attributes easier (song.measure instead of song['measure'])
• Allow the use of class methods, which would likely help to clean up some of the messier parts of the codebase

[vivaria]

An additional note: Right now in the test suite, we're manually comparing each and every property between the cached fumen files and the converted TJAs.

As an alternative, we could potentially use the __eq__ method of these hypothetical song/measure/note objects to perform comparisons between separate song objects.