om-shell and ochre

Copperbox revision 2922.

I've added a tremolo processor to OM-Shell. It is written in a slightly more "functional" style - i.e. the body of the processor is a function from signal to signal. Maybe this is a valid direction for defining instruments (processors) but the code actually isn't much shorter.

Ochre - I've fixed a very silly bug where I was using flip in Haskell for functions that built expression syntax in Ochre. Particularly, I was flipping the arguments for division, but division is not commutative so this was nonsensical.