Handle case of zero length notes created in formatters

TODO: This rounding may result in notes of length 0.

midi_degradation_toolkit/mdtk/formatters.py

Line 355 in 7038857

note_off = df.loc[:, ['onset', 'pitch']]

We had mentioned that this might be fine (even though the note_off precedes the note_on--which is somewhat important). Any model should just learn that such a case corresponds to a very short note.

Otherwise, we can just not include such notes.

We decided to force all notes to be at minimum length 1. This is because the command format is complicated by length zero notes i.e. note off followed by note on in the same time instance.