Moog ladder filter

This is a digital approximation of the famous Moog Ladder filter. 
It is based on the work of Antti Houvilainen. 
Code fragments found at:  http://musicdsp.org/showArchiveComment.php?ArchiveID=196

It has a double steep 24dB/Okt roll-off compared to the "Filter effect"
The resonance can be cranked up to self resonance, which produce a prominent sweeping effect.  

IMHO this is a nice addition to MIxxx and a good counterpart to the subtle "Filter effect" pull request, which is included in this PR as well. 

Both are designed to be an EQ replacement if desired.


It is just a hint what the following code does. 
It calculates first a parameter as if it where linear and the applies the dB Scale later. 
This is different in the audiotaper.

Any suggestion for an improved comment?