Some improvements

[syncopika]

• got some MIDI element nodes added for each instrument so that after importing, the correct instrument sounds will be used on playback (based on a static mapping though)
• took into account bass instruments for correct clef setting (not sure why I didn't have that set up before?)
• omitted instruments that are muted in the mmp file
• refactored the omission of instruments with no notes
• added new test file