group-level definition of otsu thresholds

a batch of subjects is acquired with common acquisition settings, so can be preferred, for consistency, to define a single threshold over the datasets. this should be done with group-level processing, where an aggregate histogram is created from all the datasets, and used to perform otsu thresholding (using same functions as for individual subjects). Then, this threshold could be used for all the subjects individually.. make a target rule for the group-based otsu threshold definition (e.g. all_group_otsu), and a way to use this for individual participants..