The linearity coefficients file should be a FITS file, with polynomial
coefficients for correction for each pixel stored in the data cube in
the first extension.

Note that the default coefficient file is included in the
source distribution of this package, but not in the pip or conda versions.
They may be downloaded separately, if desired, from the
`GitHub repository <https://github.com/SOFIA-USRA/sofia_redux>`__.
Otherwise, the software will attempt to automatically download and
cache the reference file as needed.
