Improve the DVC image diff workflow to support side-by-side comparison of modified images

[seisman]

Description of proposed changes

This PR improves the DVC image diff workflow, so that it shows side-by-side comparisons of modified images in a markdown table.

NOTE: commit cff85bf is only for testing this the new workflow, and will be removed before merging. In this commit, one image is added, one is removed and one is modified, so it should cover all the possible use cases.

Address #1217.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[github-actions]

Summary of changed images

This is an auto-generated report of images that have changed on the DVC remote

Status	Path
added	pygmt/tests/baseline/test_basemap_add.png
deleted	pygmt/tests/baseline/test_basemap_map_scale.png
modified	pygmt/tests/baseline/test_basemap_compass.png

Image diff(s)

Added images

• pygmt/tests/baseline/test_basemap_add.png

Modified images

Path	Old	New
test_basemap_compass.png

Report last updated at commit 7d14988

[seisman]

See #1219 (comment) for the new image diff report.

[michaelgrund]

Looks good to me!