title: Convert time/tick in Python MIDI MIDO read/save file
body: <p>The following program uses MIDO to read 'g1.mid' then save it to 'g1_new.mid'. My question is that: in reading the file, 'msg.time' is a float value, but in saving the file, 'time in Message' is an integer in tick. How do we convert 'msg.time' to 'tick in message' in this case?</p>
  
  <pre><code>from mido import MidiFile 
  from mido import Message, MidiTrack
  
  mid = MidiFile()
  track = MidiTrack()
  mid.tracks.append(track)
  
  for msg in MidiFile('g1.mid'):
      if (not msg.is_meta):
          if (msg.type == 'note_on'):
              # how to convert msg.time to tick to fill in '?'
              track.append(Message('note_on', note=msg.note, velocity=msg.velocity, time=?))
          elif (msg.type == 'note_off'):
              # how to convert msg.time to tick to fill in '?'
              track.append(Message('note_off', note=msg.note, velocity=msg.velocity, time=?))
          elif (msg.type == 'program_change'):
              track.append(Message('program_change', program=msg.program, channel=msg.channel))
  
  mid.save('g1_new.mid')
  </code></pre>
  
  <p>Note: This piece of code is in a project about music generation.</p>