symsync: adding option to recover symbol timing after matched filtering

  * uses Kaiser filter with cut-off outside signal bandwidth
  * interpolates between available sample points
  * works well; tested with Nyquist filter
filter/firinterp: adding method to create Nyquist interpolator
symsync/example: simplifying; using new Nyquist firinterp create()
filter/symsync: cleaning up code; adding descriptive comments
filter: consolidating function prototypes in global header
agc: adding method for running on entire block; needs testing
agc: setting loop filter bandwidth to non-zero value by default
eqlms: fixing bug where squared magnitude sum was not reset (thanks Ray)