study record should include time interval

[yarikoptic]

when we report from within heudiconv on what we are about to process, or finished processing -- we should also include "DateTime" consisting of start and end times for this particular acquisition (as min/max of all times from all the files). This way we might later be able to splice some other supplemental files recorded outside (stimuli delivery, electrophysiology) automatically into the correct study. That information could be gathered from .json files or may be directly from groupped/loaded dicoms