Improved image processing

Default mode will convert 16-bit images directly to 8-bit by considering
the relationship between temperature step size and image graylevel.
Absolute temperatures are not necessary for this, and temporal smoothing
ensures the central value (based on the median) does not change too
rapidly.

Default sensitivities and other parameters for optical flow have been
greatly improved. Default performance for both Optris and Miricle 307K
fairly good now.