Add +/- 1 and 3 sigma lines on residual plots

[butlerpd]

Description

Residual plots should now have 2 red lines at +/- 3 sigma and 2 dashed gray lines at +/-1 sigma. There does not seem to be any documentation that needs updating.

The only change necessary for this was to use the data1D_plot_role already established. This allows the fix to require only a small 5 line block of code in the PlotterWidget in plotter.py. Basically if data.plot_role==data1D.ROLE_RESIDUAL then add 4 horizontal lines (ax.axhline()).

Fixes #1329

How Has This Been Tested?

by running in developer environment

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.
Load a data set (AOT microemulsion for example), send to fitting, choose a model (e.g. sphere/core_shell_sphere), hit compute/plot and/or fit (after choosing some parameters to fit). The residual plots should have the extra 4 lines.

Review Checklist (please remove items if they don't apply):

• Code has been reviewed
• Functionality has been tested
• Windows installer (GH artifact) has been tested (installed and worked)
• MacOSX installer (GH artifact) has been tested (installed and worked)
• The introduced changes comply with SasView license (BSD 3-Clause)

[krzywon]

This does what it says it does on the surface, but there is a wrinkle. The P(r) plot is tagged as a residual plot so the lines are drawn on those plots as well. Fixing another plot type goes beyond this PR so I would say we document the issue and not let it hold up this change.

[wpotrzebowski]

We agreed to merge it but we need to document issue with PR plot as pointed out by @krzywon