Reorganize filter functions?

[simonster]

I want to flesh out the docs a little so that it's clearer how to actually design filters, but before I do that, I think it might make sense to reorganize the filter-related functions. Right now we have:

• FFTFilt
  • fftfilt and firfilt (should probably have mutating versions of these as well)
• FilterDesign
  • ZPKFilter, TFFilter, SOSFilter, and BiquadFilter types, conversions between types, and filt implementations
  • Butterworth filter prototype
  • Prototype transformation and bilinear transform
• FilterResponse
  • freqz and freqs
• ZeroPhaseFiltering
  • filtfilt

All of these modules but FFTFilt depend on the FilterDesign module because it defines the filter types, which suggests that this should just be one module. I propose a single DSP.Filter module that would reside in a separate subdirectory and contain the following files:

• types.jl - ZPKFilter, TFFilter, SOSFilter, and BiquadFilter type definitions. When we have FIR filter design we will probably want an FIRFilter type as well. Not sure if the conversions should live here or in a separate file.
• filt.jl - filt, fftfilt, firfilt, and filtfilt implementations (eventually firfilt will probably just be filt(::FIRFilter, x))
• design.jl - prototypes, transformations, and bilinear transform
• response.jl - freqz and freqs

Does this sound good? Any objections?

[jfsantos]

I like the proposed organization. Regarding the conversions, I would like having them in types.jl as long as it does not make the file terribly long. I find it useful to have all types and conversions in the same file.