Improve gallery example "meca.py"

[yvonnefroehlich]

Description of proposed changes

This PR contains some improvement of the gallery example Focal mechanisms.
Now, the parameter word spec is explicitly used in the code, as this parameter name is explained in the text at the beginning.

I feel, it would be good to have a more detailed example or tutorial on plotting focal mechanisms. (See also the several posts/questions on the GMT forum).

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