[PD] [iir] doesn't handle nan, -inf, inf and very small numbers well

[Hans-Christoph Steiner]

Pix and I are here working in AVLAB using [iir] for filtering data  
from [wiimote]. It turns out that [iir] gets stuck at inf or -inf if  
it receives one.  This makes sense in a math sense, but not in a Pd/ 
practical sense.

We are thinking that [iir] should filter out -inf, inf, and perhaps  
nan as well.  Seems like they would be useful objects for other  
things too.  Something like:

- filter nan        [no_nan]
- filter inf/-inf   [no_inf]

.hc

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