Bessel LV-Mix 4th and 8th order

This is a very nice and lightweight equalizer, introduces a crossover mix, described by Lipshitz and Vanderkooy.
Features: 
* Bit perfect at unity, uses a delayed dry signal.
* introduced a constant group delay of ~2 ms by default 
* Based on a Bessel8 Low pass 
* roll-off -48 db/Oct
* rounded top 
* minimum phase distortion, when not at unity 
* only half of CPU compared to the Linkwitz-Riley EQ at worst case 
* minimum CPU (a delay line) at unity