Show full plot extent using Mercator projection

[sadielbartholomew]

Fix #65, including updating the test suite generated reference image.

Note:

• The colourbar gets cut off on the image now the full-extent plot emerges after the bug fix, but this is part of a separate bug I have observed in other projections (see e.g. the existing result from the Orthographic projection from when the test suite reference images were first added), which I will do a quick write-up of now, and fix in a follow-on PR.
• I have consolidated some code around the areas I was investigating to make the fix, which is unrelated to the fix itself but useful for general code improvement going forwards.
• I fixed one docstring which quoted an incorrect default, also noticed along the way.
• I am yet to find time to set up a pre-commit system so doing linting as-and-when, so some black linting changes are included in the PR, ignore those as they are not related to the bug fix.

[sadielbartholomew]

Merging, so I can get on with other fixes as follow-on work...