majalan

Copperbox revision 3025.

I've added initial work building chords. The current representation will be insufficient to represent the Pachet chord building grammar (Francois Pachet - An Object-Oriented Representation of Pitch-Classes, Intervals, Scales and Chords), so likely it will need revising.

Also, I've fixed a long-standing, unnoticed bug with adding intervals to pitches. I seems hard (for me) to keep track of octaves in an (octave x Z12) pitch representation for equal temperament - much easier to go via in integer representation that corresponds to MIDI pitch values.