heterodyne: Use the entire trimmed spectra to estimate DC offset.

This is as requested by Jan.  It seems eminently sensible to me.
Otherwise the small overlap that might contain residual noisy ends
of each sub-band and/or a strong spectral line can skew the offset.
heterodyne: Minor tidying of the hybrid-merging primitive.
heterodyne: Update the primitive's prologue for hybrid merging.
heterodyne: Clarify the proglogue which had confused me referring to subscans instead of subbands or subsystems.

It doesn't help that there is a NSUBSCANS header.
heterodyne: Merge more than two subbands for hybrid observations.

This address Scicom Ticket #114.  It assumes that the subbands are
delivered in order.  I did not follow Brad's code where it claims to
divide the subbands into pairs, so I rewrote the pair-handling code
in a form that I could comprehend.
heterodyne: Use the fraction not the percentage to evaluate the limits.
heterodyne: New primitive to assist bad-baseline filters.

This checks for receptors containing all or virtually bad values.
in the classic pipeline this was handled by the QA stage, but
the bad-baseline filtering occurs before that, and some early ACSIS
data have such blanked receptors which causes problems in the
bad-baseline filtering.  This primitive adds an entry to the
bad_receptors_qa index for each blamked receptor, so that the later
bad-baseline filtering can exclude such planes of blank data.

Yes one could update the released bad_receptors index but it is too
incomplete and I would rather the pipeline determine the bad data
empirically than rely on $ORAL_DATA_CAL/index.bad_receptors.

This should permit over thirty jobs with processing errors at CADC
to complete successfully.