Fix BPM (off by 2x)

[nathanleiby]

ideally the data model for user_hits and desired_hits aligns nicely, i.e. 1.0 is beat 1, 2.0 is beat 2. So e.g. 16th notes are 0.25 in length

I believe today it's off by 2, i.e. bpm=120 in app is actually bpm=60.

This isn't strictly a bug as everything "works" and you can abstract over it in the UI. But it would make the internals and data model clearer if we fixed it!