[Stk] calculating a delay in physical model

[Perry R Cook]

Yep, plus more...

The loop filter has some intrinsic delay, so we subtract a tad.

Also, any delay that is fed back to itself alwas has an extra single sample of delay.  This is due to the fact that you can't know this sample until you use those samples to compute it.  If this seems odd to you just write a really short delay line on paper, feed it back into itself, and step thru the calculations to convince you that we always get an extra one "free."

ideally we'd compute the exact delay of the loop filter, but it's frequency dependent, so we just hack in some.

Prc