Internally, the radiative transfer integration is always performed with
Double Precision but uses Single Precision absorption coefficients
(since most line parameters are only known with this accuracy), hence
the incompatibility with the TAB flag.
Double Precision output is only really
necessary when considering transmittances close to 1.0 (in which case
writing absorption, = 1-transmittance, spectra
would be better),
or when comparing
different RFM runs with minor perturbations.