Fix legend remaining turned off when Waterfall is clicked

[GuiMacielPereira]

Summary of changes

• Introduced an if statement that checks a boolean flag on whether the legend is supposed to be shown or not.
• Edited the unit test of the changed function to include testing of both options, i.e. when legend is not shown and when legend is shown

Description of Work

Finding the problem

• When the legends box is clicked, a _toggle_legend function is activated which sets the boolean attribute _legends_shown to its opposite
• The attribute _legends_shown of CutPlot is thus supposed to control whether or not the legend is visible in the plot.
• The update_legend function does not make use of the attribute _legends_shown in its scope, which leads to legends being shown even when _legends_shown is set to False.
• This behaviour is present in 3 scenarios on cut viewer, as all these instances call the function update_legend:
  • When Waterfall is clicked
  • When the Intensity is changed
  • When any line in the plot is clicked

Fixing the problem

• Introduced an if statement inside update_legend
• Same if statement is used in the analogous slice viewer

Edited unit test

• Introduced the new scenario of making sure axes.legend() is not called when _legends_shown is set to False.

To test

• Open cut viewer in mslice and run over the 3 scenarios that were previously making the legend visible:

  • Click Legend so legends become invisible
  • Click Waterfall and check legends remain invisible
  • Click any plot line > cancel, and check legends remain invisible
  • Click Intensity > any intensity that works, and check legends remain invisible

• Comment out if`` statement and run testcut_plot_test.py. It should fail the testtest_update_legend. Restoreif` statement and check the test passes.

Fixes #863 .