heterodyne: Restore original formula to find peak value.

FINDCLUMPS was working fine in respect of returnuing the peak bin.
The main two bins were close in value and the smoothing had flipped
which of the two was the peak, only emphasing the need to find the
centroid or increase the resolution.
heterodyne: Iterate to test for non-normality.

Sometimes the clipped statistics of raw data pass the albeit arbitrary
test while clearly skew.  This adds a loop so that the histogram is
also tested for wings.
heterodyne: Attempt to improve centroid by removing outlier signal.

This works in that the outlier signal is relatively diminished compared
with the main peak, however centroiding is only improved slightly.
heterodyne: Turn on history reporting for the histogram NDF.

This was added to help with diagnostics.
heterodyne: Initialise maskspectrum for non-automatic emission masking.