biquad filter sketch (DON'T MERGE)

I suggest to add a biquad filter like this.
With this we can implement lp/bp/hp filters.
It is computationally simple and there are a lot of resources on how to dimension biquad filters.
We could use this filter for the PID derivative filtering.
What do you think?
+ f->a1 
should be 
- f->a1

same for line below