heterodyne: Step fixing not used for low-frequency interference.
heterodyne: New primitive to find the histogram peak.

I needed this to determine better thresholds to remove more non-linear
baselines.  The sigma-clipping was not adequate in more extreme cases,
such as we have had recently with H06 and Glenn White's M17 data.  I
had already used similar code to correct baselines in line forests for
DAS-era data.  So I refactored and added some tailoring options.

It does use clunking search of the catalogue rather than preferred OO
methods to access the relevant data from the clumpfinding.
There are no astronomical co-ordinates whose clump properties I seek,
so Astro::Catalog is not relevant.
heterodyne: Revert to copying a temporary NDF rather than name assigment.