original_fnames: for resampling
The files to resample. 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", "surfL", or "surfR". If
read_dir
is not NULL
, then all these file names should be
relative to read_dir
.