heterodyne: Velocity range undefined only applies to sum method.
heterodyne: Fix bugs in flat fielding.

-  The wrong index was being used for the reference receptor,
   presumably arising where the loop was made go through all 16
   receptors and not just the available ones.

-  In itersum method collapse with the sum not the mean as the
   threshold can include a variable number of pixels which can
   over-accentuate receptor-to-receptor differences.  This can lead to
   divergence rather than convergence for one or two receptors in the
   iterations.  This has only been tested on a couple of cases and
   needs to be checked on some earlier examples where the divergence
   had originally been noticed.
heterodyne: No longer need the full flatfield ratios.
heterodyne: Report the internal errors in the flatfield ratios.

Yes these now appear in the log.flatstats, but it seems useful to
present them in the ORAC-DR log too.  They are likely more visible
there.
heterodyne: Declare commasep after reintroduction.

Also correct commasep comment to match the code.  This completes
2804a20275fab.