Beginning refactoring of biquad API. The end goal is to have biquad as a
part of pulsecore with a reasonably simple way of managing filters from
pulse or modules. 

The initial change is to move the filter factors (coefficients), filter
type marker and filter history data from the user into biquad.c/h. All
filter related items will live in the filter map array. 

Let's hope this is a coherent plan. :)