PR 65624: Add another term to the dispersion relation in CALSTIS.
A ninth term has been added to the dispersion relation. This is
proportional to m**2, where m is the spectral order. If wl is the
wavelength in Angstroms, the dispersion relation is:
pixel = coeff[0] +
coeff[1] * m * wl +
coeff[2] * m**2 * wl**2 +
coeff[3] * m +
coeff[4] * wl +
coeff[5] * m**2 * wl +
coeff[6] * m * wl**2 +
coeff[7] * m**3 * wl**3 +
coeff[8] * m**2
