Consider revision of FIRuncFilter

[mgrub]

Currently, FIRuncFilter allows to hand over a low-pass blow to pre-filter the signal before the actual FIR-application. This is in line with the underlying paper.

Because blow is just a another FIR-filter, I could image a re-implementation that recoursivly calls FIRuncFilter if blow is given. This should reduce the complexity of the code. However, this would then deviate somewhat from the equations stated in the underlying paper.

Also, rigorously testing is required, so that we can assure that this is just a refactoring.