Investigate thickness mapping issues

[cmeyer]

ND:

I tried the thickness mapping on [redacted], it really has artifacts making it
look ugly. See attached

None of the artifacts show up if you just display the ZLP intensity or
the loss intensity.

So it seems that the artifact source is from changing which channels are
used for the ZLP extraction from pixel to pixel, see attached image, as
things get thicker you presumably start using more channels as being
part of the ZLP, and when you add one channel more gets counted as ZLP
making for a discrete jump down in thickness.

So if you just make the channel choice global processing is 1000x
faster, and the map will look much better.

Ref: email 2021-04-20.