heterodyne: Threshold for noise spikes about the median.

For most data this is not necessary but there are about forty failed
jobs in the CADC processing because the data have a high base level,
causing all data to be rejected and a crash further down the line
(division by zero in QA).
heterodyne: Invoke the new primitive before bad-baseline detections.
heterodyne: More thresholding for outliers about the median rather than assume a zero base level.
heterodyne: Improve the commentary.
heterodyne: Add steering to pre-filter receptors comprising blank values.

This means that the checks are only made if any bad-baseline filtering
is selected.
heterodyne: Tidy bad_receptors selection for bad-baseline identifications.

Remove superfluous lines now that MASTER is no longer the default to
select the bad receptors in these primitives.  Add some comments about
the command-line -calib bad_receptors.  We want users to avoid case
where the MASTER will exclude unrelated bad receptors and retain known
bad receptors.  INDEXORMASTER still has the issue of the grossly
underpopulated $ORAC_DATA_CAL/index.bad_receptors.  Therefore the
default in these primitives will now be INDEX.  Recall that this is
different from Luca's original which selected MASTER to ignore
receptors and INDEXORMASTER to setup up the survey to augment the
index.  INDEX should provide the latter.