Fix interpolation and ranging

Verified with gdb inspection only, but this is much closer to viable now.

Still could do with optimisations to interpolate into a shorter wavetable for
higher speeds, at 512 samples, we're only getting about 2khz for sine waves
with a 1msps dac.

Still needs support for the L1, still needs support for full custom wave forms
downloaded via usb, still needs a lot of things :)
Update directories and readmes to accurately reflect their purpose