Significant refactoring and cleanup. Created
src/modules/biquad/biquad-filter.[c/h] to hold generic biquad filtering
in preparation for creation of module-xover-sink as discussed with
Alexander Patrakov. 

Updated module-lfe-lp and Makefile.am to use these new files.

Also changed formatting and documentation of module-lfe-lp to begin to
comply with pulse' stype guide.