heterodyne: Protect against FINDCLUMP failures to locate histogram peak.

FINDCLUMPS has been observed to fail during testing.  I had left this
in place while adjusting the recipe as it would be more evident that
something has gone wrong.  Given that now the current arrangement is
working reasonably well incorporate a status check for FINDCLUMPS too
defaulting to zero clumps.  Improvements elsewhere in the primitive
have lowered the chance of a crash, but it is always a potential risk.
In particular the clump parameter settings such as contour base level
and increment, and minimum number of pixels do need some more
consideration in the light of anomalous or pathological data, or
indeed the size of the sample.  Until I have time to look at this
again it's prudent to offer protection against FINDCLUMPS yielding an
error status.
heterodyne: Push some intermediate files so they may be tidied.

One primitive was easy, but the other one
(_FLAG_LOWFREQ_INTERFERENCE_SPECTRA_) required a change to the name
of the subsys subgroup Frame, otherwise the Frm->push_intermediates
applied to the subgroup Frm not the master Frame propagating through
the recipe.
heterodyne: Attempt to filter noisy spectra from short time series.

This addresses Tim's 19951126 data which had a single anomalous
spectrum amongst the 59 time series.  In an effort to provide a
sensible noise level above which to threshold noisy spectra I had
imposed a limitation on the minimum number of spectra to compare in
order to make FIXSTEPS and FINDBACK work satisfactorily (100 for a
50-pixel wide filter).  The lower limit of 7 (14 spectra) is a number
plucked out of the air.  We might well revise this after processing
more DAS data.