Midi header writing finished

The time division (finally understood) is now correctly written and corresponds to ticks per beats.
Small naming errors corrected in Intervals definition
Interval naming errors totally corrected
Error corrected in BarFactory class

Some TimedElement instances were not copied.
MidiWriter update

The MidiWriter class is now able to write a file containing NoteOn and NoteOff events and having several tracks.
Main class update

The program now generates 4 bars of random music.