Accept `xfms/standard2diff_warp` and `xfms/diff2standard_warp` for non-linear warps
This MR updates xtract
so that the default value for the -stdwarp
option will be set to xfms/standard2diff_warp.nii.gz
and xfms/diff2standard_warp.nii.gz
or xfms/standard2diff.nii.gz
and xfms/diff2standard.nii.gz
. The FDT GUI saves non-linear warp files as xfms/diff2standard_warp.nii.gz
/ xfms/standard2diff_warp.nii.gz
, so it makes sense for XTRACT to accept these file names.