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. 
Nice. But to be honest I have a hard time understanding your comments because I'm missing some background. I'm happy that yo found and error and trust you to fix it.

It would nevertheless be nice if you find some time to document this a bit in the wiki, at least showing where we can find the information. For example I don't know what you mean with the 'infinity nature'