Swath accuracy: uncertainty is shown as zeros (when not parsed)

[kjerram]

Reference surface uncertainty is shown as zero when it is not parsed (e.g., from an average Dynamic Surface export to .xyz, rather than a CUBE surface export to .xyz).

Update plotter so that the uncertainty plot is not shown (uncheck 'Show uncertainty plot if parsed' option) and add note in activity log for user re: no uncertainty data available. Re-enable / re-check this option if uncertainty is parsed.