dmxgus: Add comparison MID generator script.

This script generates a MIDI file that plays all of the comparison
groups from config.py. This allows instruments to be compared, so
that similar-sounding instruments can be grouped together.
dmxgus: Tweak similarity groups for main instruments.

The main instrument groupings were already pretty good but there
was some room for improvement. The generated configuration sounds
pretty good now, even when using the 256KB config.