Issue/analysis notebook

[ghalestrilo]

This branch:

• Fixes a bunch of time-related issues for the magenta modules
• Refactors a big part of analysis notebook code
  • Uses commands instead of a separate server
  • Stores models and checkpoints in a DataFrame for simplicity
• Sketches test code (for the future)
• Makes the engine wait for each message's .time duration before playing it - I think this is how engines work