pygmt.set_display: Fix the bug that method=None doesn't reset to the default display method

[seisman]

Description of proposed changes

pygmt.set_display(method=None) should revert the display method to the default value, but it doens't work. Here is a minimal example to reproduce the issue:

In [1]: import pygmt
In [2]: fig = pygmt.Figure()
In [3]: fig.basemap(region=[0, 10, 0, 10], projection="X4c/4c", frame=True)
In [4]: fig.show()  # Display the image in an external viewer
In [5]: pygmt.set_display(method="none")  # Disable image preview 
In [6]: fig.show()  # Image is not displayed
In [7]: pygmt.set_display(method=None)   # Should revert to the default display method
In [8]: fig.show()  # Image is not displayed. WRONG!

In the last fig.show() call, the image should be displayed, but it doesn't. It's because method=None is ignored in pygmt.set_display.

This PR fixes the bug and does some refactorings.

• 2409284: Add the _get_default_display_method function so that the same logic of codes can be called when method=None is used.
• 6bd742e: Refactor set_display to fix the bug.

Ideally, we should add a test to check if SHOW_CONFIG["method"] matches the value in pygmt.set_display(method="XXX"). However, SHOW_CONFIG is a global variable, changing its value will affect other tests. So, no test is added.

[seisman]

Need to wait for #3418.

[seisman]

Ping @GenericMappingTools/pygmt-maintainers for final review.

[weiji14]

We don't often use classes for the pytest unit tests, is there a reason to use it here?

[seisman]

Just want to group multiple tests so that it is easier to find all tests for testing a specific function.

[weiji14]

Ok, it looks like we're using TestGetDefaultDisplayMethod below also, so ok with this.