Add (commented-out) section on noise model.

Discuss impact of non-white detector noise on image NETD. But getting
this to the point where I really trust my results is too much work for
now, so I'm not ready to really include it in the thesis. Saving it in
comments in case I get more confidence in in later.
Image Noise Model section now complete.
Rthermal load from BOSE wires belongs in ch 4
Need a label for common mode section
Editing ch8.

Spelling. Clear 'xxx' comments. Fix references. Drop duplicate words. Get
rid of bad weasels.