Added support for biquad, parametric, and multi-band filter eq

Feature requested by Philip Rosedale.  Have implemented a multi-band parametric eq, and have integrated it with the interface client app.  Two new files, AudioFilter.h/.cpp contain the actual DSP and glue-code for the client application.   The design is completely general purpose, and may easily be adapted to support new high/low pass, high/low shelf, all pass and other biquad filter types.  See http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt.   Only a parametric/peaking EQ design has been implemented here (with support for variable center frequency, gain, and resonance/q).

This proposed design also makes makes it easy trivial aggregate large banks of filters by using c++ templates.  See template< typename T, const int N> class AudioFilterBank.

To test/enable the feature, enable:
  'Developer:Audio Debugging Tools:Audio Filter Bank'

Four sample 3-band parametric filter profiles response curves have been created:
 ' 'Developer:Audio Debugging Tools:Audio Filter Options:Flat Response'
 ' 'Developer:Audio Debugging Tools:Audio Filter Options:Treble Cut'
 ' 'Developer:Audio Debugging Tools:Audio Filter Options:Bass Cut'
 ' 'Developer:Audio Debugging Tools:Audio Filter Options:Smiley Curve'

Craig Hansen-Sturm