change the filter dx in R

Now the subjects missing dxchange will be removed before grouping
Bet and flirt implemented
changed traverse to function
Finish Implementation

Encountering problem:
'''
Demon Registration (BRAINS) terminated with an exception:
itk::ImageFileReaderException (0x75e2bd0)
Location: "unknown"
File: /home/kitware/Dashboards/Nightly/Slicer-0-build/ITKv4/Modules/IO/ImageBase/include/itkImageFileReader.hxx
Line: 143
Description:  Could not create IO object for reading file slicer:0x2583f10#vtkMRMLLinearTransformNode2
The file doesn't exist.
Filename = slicer:0x2583f10#vtkMRMLLinearTransformNode2
'''