heterodyne: Find the spectral bounds of the NDF to be used in MFITTREND.
heterodyne: Limit to RASTER and SCAN observation types.

Doubt that even jiggle will generate worthwhile flat field ratios.
Certainly stares do not and can go haywire in one of the receptors
exhibits strong non-linear baselines.
heterodyne: Ensure that the degrees of freedom exceeds one.

Entire receptors could be rejected during the iterative phase, so
prevent divide-by-zero, when only one good receptor remains.  Having
one rather undermines the receptor-to-receptor flat field.  This change
is protection again pathological data.  Now that stare observations
not accepted (7acd199a971ac), this may be superfluous.
heterodyne: Stop the threshold from growing too much.

When it grows it is usually a sign that there is some crap data
biasing the threshold, and hence sensible data being rejected, further
raising the bar in a runaway.  The 10% increase is a bit arbitrary but
you would not expect after an iteration correcting for the first-order
receptor sensitivities that the mean threshold would rise by more than
this.  In the light of experience it might need adjustment to 12 to 15%.
heterodyne: Protect against divide-by-zero while scaling errors.