BiCal dialog box:  added a "Determine" button for the "Air Threshold" parameter.  This invokes the dialog "Split ROI by threshold", so user quickly manually adjust the "Air Threshold" parameter and it is remembered.

"Split ROI by Threshold" functionality - last accepted threshold setting is now remembered.

"Split ROI by threshold" menu command - expanded to include 3 options for the initial segmentation:
a) Split by specified Threshold
b) Split by the portion (50/50)
c) By adaptive split of the histogram

"ROI Split by Threshold" dialog:  expanded functionality so the ROI is not required, in which case the entire volume is thresholded.