v1.15.2 -- a) Fix bug where if snirf files weren't successfully loaded...

v1.15.2  -- a) Fix bug where if snirf files weren't successfully loaded because they're in obsolete format and you cancel instead of choosing another folder the GUI would keep asking in an infinite loop if you wanted to choose another group folder. b) Change DataTreeClass.group to DataTreeClass.groups to reflect that it one or more groups now because Homer3 supports loading multiple groups.