v1.12.3

-- Added error checking for acquired data to SnirfClass. Discard data that failed to load.
-- Got rid of SnirfDataClass and NirsDataClass and moved all file loading error checking to DataTreeClass.AcqData2Group() and to the SnirfClass and NirsClass the same classes that are used for acquired data.
-- Rework listboxFiles display to listboxGroupTree display in Main GUI to be more intuitive. Change the behavior of processing level GUI controls and displays to more closely follow group tree structure which now also displays the root group folder. Add multiple group tree display views: All, Subjects, Runs.
-- Added ability to load SNIRF subfields to subclasses like StimClass independently.
-- Update README.md to include Introduction section