original_fnames: for remapping
The files to remap. This is a named list
where each element's name is a file type label, and each element's value
is a file name. Labels must be one of the following: "cortexL", "cortexR",
"ROIcortexL", "ROIcortexR". If read_dir
is not NULL
, then all
these file names should be relative to read_dir
.