Update existing inline examples to the new doctest style

[seisman]

Description of proposed changes

Update inline examples to the new doctest style in #1790. Also fixes problems when running these doctests using pytest pygmt/src/*.py.

Address comment #1686 (comment)

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 wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• 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

[seisman]

The original example gives the following error:

grdgradient [ERROR]: Cannot specify both -A and -D

so I update this example.

But I'm using GMT dev branch.

[maxrjones]

Yes, I added that error following the discussion in GenericMappingTools/gmt#6327. It is good to update it.

[seisman]

When I run pytest pygmt/src/*.py, it seems all doctests run twice. I don't understand why.

[maxrjones]

When I run pytest pygmt/src/*.py, it seems all doctests run twice. I don't understand why.

I think pytest collects each item once from pygmt/src/__init__.py and once from each other module.