Vocoder 2.0

The main innovations are twofold: decoupling the vocoder code into several steps (calcuate freq bands, bandpass the signal, generate envelopes, generate carriers, reconstruct), and adding a poisson click train as a built-in option for the carrier.  Have a look, @rkmaddox, and see if (1) it works as expected, and (2) it is organized in a sensible way.  I'm happy to do some refactoring to make this optimally useable.  @Eric89GXL feel free to chime in too.  

No new tests as yet, and since I changed the name from vocode_ci to just vocode, the old tests will fail at the moment.
you should just be able to do `filt = filtfilt if zero_phase else lfilter; out = filt(b, a, x, axis=axis)`