heterodyne: Document the trim-percentage recipe parameters.
heterodyne: Check and report each hybrid subband independently.

For hybrid data it is possible to have one subband mostly bad but
others not.  If combined into one large dataset, as if concatenating
subscans, the badness is diluted, and so the receptor's bad subscan is
retained.  This can cause problems with merging, as such data seem to
have weird non-bad perpheries (cf. 20140210 Obs 20 Receptor H01
(Subband 1) & H06 (Subband 2), and 20140213 H01 (Subband 1).

For now at least take the approach of excluding the entire receptor if
one of its subbands is mostly bad.  Adjacent good receptors could be
retained provided the suspect subband is made entirely bad before
merging is attempted.
heterodyne: Add FRACTION argument.

This is also a recipe parameter FRACTION_BAD.

Adding this tuning proved necessary because some suspect hybrid
subscans have around 80% bad values with other crazy patterns and
values outside of the bad values.  The artefacts include alternating
bad stripes at the ends (but extending inside the normal noisy edges
of spectra), and a period of strong non-linearity.  These need to be
tackled before merging otherwise artefacts appear in the composite
spectra.  So the normal 0.999 it far too tight.  I had thought that I
had reduced this to 0.9 or at 0.99 months ago, but now do make
the default maximum fraction a sensible 0.9.
heterodyne: Add FRACTION_BAD to steering primitives.
heterodyne: Document new FRACTION_BAD recipe parameter.
calib: Allow combinations of a list and another option for bad_receptors.

Jane Buckle wanted to supplement the default bad-receptor method on
the command line, not replace it, when specifying a list of receptors
to exclude.  The same colon-separated syntax is used.  For instance it
is now possible to write "-calib bad_receptors=H01:H06:index" to
exclude H01, H06, and any others found in index.bad_receptors_qa.
heterodyne: Update from the working primitive.

Looks like I failed to copy the working primitive from the test
$ORAC_PRIMITIVE_DIR to my clone before making Commit 2cac7a110286f.
It was used with a 0.75 threshold to reduce Jane Buckle's CN
hybrid observations successfully.
heterodyne: Tweak heading underlines for X output.

Not sure if upgrades after a system reboot and a slightly
differently spaced fount appearing in terminals has affected these.
I'd like a better underline method that could work out the required
number automatically.

Some changes were for primitives ones where I had only committed an
approximate first estimate without evaluation and refinement.