Add support for chirp generation to the DSP package

[glopesdev]

In general having a chirp generator would be useful as a basis for calibration and other auditory parameter reconstruction.

A minimal implementation should probably have support for both linear and exponential chirping as described in https://en.wikipedia.org/wiki/Chirp#Types.

Note this cannot currently be implemented as a variation of the FunctionGenerator operator since chirping requires specifying a frequency range, rather than a single frequency. We might be able to start without explicit support for continuous generation, as the goal is often to directly generate a full sweep waveform.

If we go down this route, however, we might want to simply integrate and wrap something like NWaves.