Added template based IIR base class
This allows to create most of fidlib filters.
Added a Butterworth4 filter to test it out.
Move setFrequencyCorners content to base class
used IIR base class for Bessel4 and Butterworth8.
Repaired ramping (regression from last commit)
Speed optimized ramping
fixed typo
fixed typo in include guard
Introduced enum IIRPass
fade input of the new filter, to start from a settled bbuffer state.
Improve comments and wrap long lines
Reverted the Input fading and introduced a ramp-delay of 1/2 buffer size
This avoids the gain drop during filter change, and is a good compromise between a setteld filter and latency.
Merge pull request #294 from daschuer/iireqs

IIR EQ base class