Figure.meca: the 'scale' parameter can accept int/float/str values

[seisman]

Description of proposed changes

Currently, the scale parameter doesn't accept int or float values like scale=1.0, because we use kwargs["S"] = data_format + scale to build the -S argument. int/float scales are valid and should be supported.

This PR fixes the issue. The test_meca_spec_dict_all_scalars is updated to catch the bug.

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.
• Use underscores (not hyphens) in names of Python files and directories.

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