Add documentation and TODO concerning meaning of total_time field in … (#1158)
Add documentation and TODO concerning meaning of total_time field in … (#1158)

* Add documentation and TODO concerning meaning of total_time field in NoteSequence.

* Respond to reviewer comments

magenta/magenta,jesseengel/magenta,jesseengel/magenta,adarob/magenta,magenta/magenta,adarob/magenta