_splat.c: add basic overtones signals implementation
signal_bm.py: add 'mixed' for overtones case
_splat.c: use tuple and add phase to overtones
signal_bm.py: update overtones with tuples
_splat.c: simplify max ratio in overtones
_splat.c: add phase to triangle
_splat.c: add phase to square
signal_bm.py: improve modulations and -o option