Update to clang-format-5 and reformat the codebase 

Refactor the method to simplify it, taking advantage of HistogramData's
functionality.

re #16798

re #16798

Histogram data?
  (yes) - Distribution data?
                (yes) - Frequencies
                (no) - Plotted as a distribution?
                             (yes) - Counts [convert data here]
                             (no) - Counts
  (no) - Counts

re #16798

This option (View/Preferences/2D Plots) plots the data as a distribution.

If this option is set or the original data was a distribution, use frequencies.
Otherwise (option not set and not a distribution, or point data) use counts.

re #16798

Instead of inserting y data from spectrum 0, just copy the previous
user-selected background point. This works better if spectrum 0 has
very different data (for example if it is a monitor) and also is less
surprising behaviour.

re #16798

Update to use new HistogramData and ensure it is correct for histo and
point data

re #16798

re #16798

The zero rows at the end are from blank rows (see #16807) but other
rows with (0, 0) are real points that just happen to be zero.

re #16798

re #16798

CreateUserDefinedBackground takes a set of points that the user has chosen
and creates a background workspace out of them. It interpolates the points
so the resulting background can be subtracted from the original data.

re #16798

re #16798