Add adjustment of tempo/tick scaling in adjust_times
Add adjustment of tempo/tick scaling in adjust_times

This will ostensibly make it so that the tempo mapping in the MIDI
object remains valid after calling adjust_times (e.g., get_beats() will
still return the correct thing).  This is accomplished by adding tempo
change events at each place the new time mapping dictates that the tempo
should change.

douglaseck/pretty-midi,craffel/pretty-midi