Implement mixer stage in HDL

* Move HDL parameters into a dictionary.
* Poly-FIR interpolator, decimator, and rational resampler.
* DDS Spectrum Mask Test.
* Enhance DDS to become a mixer.
* CIC Interpolator Impulse Response Coefficient algorithm.
* Improve computation of the simulation output FFT.
Merge branch 'master' of https://github.com/testaco/whitebox into feature/sf2

Conflicts:
	hdl/fir.py
	hdl/whitebox.py