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. 
Here is the link to the paper:  
http://quod.lib.umich.edu/cgi/p/pod/dod-idx/elimination-of-transients-in-time-varying-allpass-fractional.pdf?c=icmc;idno=bbp2372.1995.096

The essence of it is if you wish to fade the outputs in a math correct way, you have to run the new and the old filter all the time. This is the drawback of the infinity nature or a IIF. 

> With the old code it was clear that we did a linear change

This is correct for the abrupt gain change probably introduced by the new filter. 

But there is an other source of noise: 

We can compare it to a analog OP filter, witch will produce a crackling when you switch it on, and feed it with a rectangular pulses, which contains many unwanted frequencies in its FFT. If we fade the output, the crackling is still there, and mixed to the resulting signal but with a low gain starting at zero. That is the reason that the noise is reduced a lot compared to an unfaded version. 
If you instead fade the input, you get a smooth power up. There are still unwanted frequencies in the FFT of that triangular impulse, but much less. The best start up waveform would be a Hanning window (Cosinus form), but this involves more math. A triangular impulse is IMHO a good compromise.