heterodyne: Introduce FINE_RESOLUTION argument.

This controls the width of bins.  It allows for the case when the
approximate peak is adequate or where the data are such that multilple
peaks will emerge at finer resolution, such for bad-baseline outlier
removal.
heterodyne: Invoke the revised arguments of _FIND_HISTOGRAM_PEAK.

Choose appropriate resolution and method.
heterodyne: Use the sum of the histogram to rescale.

This is for the over-populated peak bin test that decides whether or
not to set a finer resolution.  Before it incorrectly checked against
the original number of good values.
heterodyne: Move the histogram smoothing before obtaining histogram statistics.

Noise spikes and multi-modal histograms on an overall smooth peaky
distribution can give a false high reading increasing the resolution,
which may exercerbate the issue.  Smoothing first will offer a better
idea if the bins are too broad in general too.
heterodyne: Rework smoothing in  _FIND_HISTOGRAM_PEAK_.

Rather than move the smoothing, what was needed was additional smoothing
at the stage where the peak bin size determines if the resolution is
too coarse, as well were it was previously.
heterodyne: Documentation update.
Merge branch 'master' of github.com:Starlink/ORAC-DR