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.


needs ```tr()```

Also, the text is awkward when you say what it lets you do. Rather, prefer describing what it does declaratively:

"A 4-pole Moog ladder filter."