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. 
The old code here does

```C++
pOutput[i] = new1 * cross_mix  + old1 *  (1 - cross_mix)
```

You apply `cross_mixxx` already to `pIn[i]` when you call `processSample`.  Does this still produce the same result for `pOutput` or was the old way flawed?  If both ways should produce the same end result I prefer the old way because it is much easier to understand.