Create a functional abstraction for measures

[oubiwann]

Foundational, non-MIDI support for this will be done here:

• Add measure support undertheory#19

Tasks:

• support playing arbitrary voices
  • examine polyphony support
  • come up with default behaviour when voice count is exceeded
• support play-count / repeat capability per-measure
• utilise measure metadata for
  • making measure-level time sig changes
  • making measure-level key changes

Supporting / consistency tasks:

• change current / existing logic that calls this "repeat" ... it should instead be called something like "play count"

[oubiwann]

I think most of this work should actually happen in the undertheory project, and then those new features can be used here in undermidi in combination with actual MIDI operations.

I've updated the description to include just the things that should be done here.