HNotate, Bala and ZMidi

Copperbox revision 381.

I've had not internet connection since Wednesday so this is a bigger update than usual.

HNotate - I've added a beam grouping level in the abstract syntax tree. Adding beam grouping into the AST has made the beam handling code better - previously beaming was essentially lexical - when the outputting functions saw beam_start or beam_end in the sequence of notes, rests and mark, the functions changed their behaviour accordingly (they were stateful). Now that beam grouping is in the AST the outputting functions are simpler as they can use normal top-down traversal strategies.

Bala and ZMidi - I've added -Wall annotations to the files.