CropReg: adjust intensity when asking for initial crop, and handle limit on frames.

* add option to set limit of frames to 0 to select all.
* adjust image intensity when displaying to request initial crop
* use [~, m] = max (), instead of find () to find maximum of normxcorr2.
* update requirement to image package version 2.2.0 instead of the
unoffical development version 2.1.1.