Refactor Figure.meca() to better determine the data format code

[seisman]

Description of proposed changes

The Figure.meca() method is too complicated to read and maintain, and also has the lowest code coverage (~75%).

I will try to refactor the method, but in separate PRs. Otherwise the changes would be too big to review.

This PR is the first step to refactor the codes. The old code uses nested if-else-statement to determine the data format (meca -S option) from parameters convention and component.
The new code uses two dicts instead, and also wrap the code in a separate function data_format_code , because the function will also be used in Figure.coupe() (not wrapped yet).

I also add several doctests to check the function and increase the code coverage.

References: https://docs.generic-mapping-tools.org/latest/supplements/seis/meca.html#s

Fixes #

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

[seisman]

Ping @tjnewton @liamtoney for review as you wrote the initial Figure.meca() method in #516.

[tjnewton]

Ping @tjnewton @liamtoney for review as you wrote the initial Figure.meca() method in #516.

Good idea. I like your changes.

[liamtoney]

Nice.