IIR EQ base class

This introduces a common base class for all fidlib based Filters.
It makes use of templates to be generic as possible, without introducing CPU overhead. 
We have found out, that a filter only works as desired if it is slatted. 
My first Idea was to wait (ramp-delay) until internal buffers are filled e.g. after 16 Samples. But this was not enough since we have an "Infinite" Integration Filter. A method to check when it is settled, is to process an Impulse of one sample and check when the buffers are again near 0.  The result depends on the the corner frequencies and can be up to 1000 or more for low frequencies.

I have discussed this with Vesa Välimäki a Prof. from Helsinki who has filed a paper about it. And we came to the conclusion that it is best to fade the Input from 0, because a filter with all buffers Zero is already in settled state for all input samples = 0. 

On the other hand it is required to fade the output samples of the old filter, because it would be only at or near Zero 1000 Samples after we are zero at the input.  

I think I will extend the comment a bit.