Figure.meca: Fix beachball offsetting for ndarray input (requires GMT>=6.5.0)

[seisman]

Description of proposed changes

Changes in this PR:

1. 96b5c51: Rename function data_format_code to convention_code and update docstrings
2. 5e93de0: Add a new function convention_name which determine the convention name (e.g., "aki") from convention code (e.g., "a")
3. 6de0907: Add a new function convention_params to return the list of focal parameters
4. 7ae8816: Add a test for offsetting beachballs with ndarray input based on beachball offsetting doesn't work for pandas/dict/ndarray input #2016
5. a87c0c5: Refactor the Figure.meca method to make it work with ndarray input.
6. cc74814: Skip the new test for GMT<6.5, because the new test requires the upstream fix Do no free external trailing text strings gmt#7557.

Offsetting beachballs for ndarray input requires GMT>=6.5, due to an upstream bug fixed in GenericMappingTools/gmt#7557.

Fixes #2016

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.
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