dsp math dependencies

[wsc1]

The module project-level dependencies firmly place dsp as needing sound. Currently, this is only for the frequency type and fft frequency-bin functions. The rest is just math with floats at this point. Eventually, as we add real time STFT support and related functionality, and controllers for the dsp functionality, we will need at least sound.{Source,Sink}, ioval meters and controllers, and maybe even plug.

It may nonetheless be cleaner to place the pure float64 math functionality apart.

Thoughts appreciated.