heterodyne: Protect against FINDBACK failing.

When FINDBACK fails, attempt an equivalent block smooth, where the
smoothing is necessary.  For the high-frequency interference
filtering, this is believed not to be required, where in effect we
skip the FINDBACK step.  The main peaks do stand out clearly for the
first filter.  For the ringing, a block smooth make incorporate the
signal being detected and after subtraction weaken the filter.  The
ringing filter is still not finalised so leave consideration of a
FINBACK alternative until further analysis and refinement of the
ringing filter.
heterodyne: Dynamically adjust the trimming to reain the minimum overlap.

There are observations where strong spectral lines lie in the overlap
region.  The default method of using MFITTREND to determine the
baseline ranges and compare these for overlap fails in such cases.
The alternative had been to use the supplied trimming percentages,
even if these removed all overlap.  Then current change reduces the
trim percentage to enable the minimum overlap in pixels.  This is far
from ideal as the DC correction can be misplaced because of the few
pixels in a strong and noisier (in absolute terms) region.  Hence
there is commentary made.  I dithered over making it a warning
as it might induce a Fault from Harriet when such instances are
reprocessed, and the offset baselines are evident.
heterodyne: Always use the HYBRID variant for hybrid data.

There many recipe instances where there are both few spectra and the
default overlap is less than twice the default trim percentage,
preventing subsequent merging.  Even if this will add some additional
(but quick) processing, it is better to have a merger than none.  So
call the HYBRID variant not the vanilla trimming, as the HYBRID method
can now handle these cases by reducing the trim percentage dynamically.
gnu: Correct the licence typo's.
heterodyne: Ensure that the spectral axis attributes are correct.

Some early data were taken with the Frequency (Hz) set rather than the
normal Radio velocity (km/s).  Without this correction, recipes such
as REDUCE_STANDARD that use velocity-range sections access only bad
values.
heterodyne: Allow for some DAS data with missing usedetpos.