filter: adding ccc, ccz (complex double taps), ccf, and ccd IIR filters.

Renames grc file filter_iir_filter_ffd.xml to filter_iir_filter_xxx.xml

More proof that templates are practically worthless. complex<float> * complex<double> doesn't work and has to be specialized. Forced the creation of iir_filter.cc.
Merge branch 'master' into next
docs: more documentation.

Stealing some of Martin Braun's PMT tutorial work for the pmt page. Added more info on constellations.
docs - pmt
Merge branch 'maint'
Merge branch 'master' into next