heterodyne: Have another go at handling anomalous data before moment-map creation.

Main thing is to check that the median rms is not zero as this is used
to set the contours in FINDCLUMPS.  Looking for low values eliminates
many good data---not sure what I was thinking there and can only
suspect that I must have mixed it with $medianrms.  The additional test
of the values may no longer be neccessary because if the median rms
should be zero when all the values are the same.

Add commentary when the median rms is invalid.

Also space a couple of related lines to improve readability.
heterodyne:  Try to use autogrid correctly making the group cube.

While the previous feature change (5d4b54096417) accidently fixed the
Fault 20110519.002 casued by (821bf061efdc) because it used "autogrid"
rather than "autogrid=yes" to lower the chances of exceeding the
maximum command-line length, and so was not being substituted later.
For now only reset autogrid=f for chunked data.

Note I have not checked yet whether the original problem with MAKECUBE
and certain chunked data continues to work correctly after these changes.
heterodyne: Prevent divide by zero in RMS-uniformity QA test.

Also write an appropriate logging message if the rms percentage is
undefined.