Fix LN_COLUMNS for single slice nifti data

[layerfMRI]

For LAYNII versions v1.5.6 and older, the programs LN_3DCOLUMNS and LN_IMAGIRO do not work with single slice data.
The current work around is to stack three identical slices on top of each other with fslmerge
Future versions should not need the hack and the programns should work with single slice data too.