clib: Wrap the GMT API function GMT_Read_VirtualFile

[seisman]

Description of proposed changes

Wrap the GMT API function GMT_Read_VirtualFile which can read the data from a GMT virtual file and cast it into standard GMT data types (e.g., GMT_DATASET and GMT_GRID). This API function will be used in supporting GMT_DATASET/GMT_GRID data types in PR #2398 and #2729.

The same codes are already used in PR #2729 and #2398. This PR is created to make those two PRs smaller and easier to review.

• API documentation: https://docs.generic-mapping-tools.org/dev/devdocs/api.html#gmt-read-virtualfile
• C source code: https://github.com/GenericMappingTools/gmt/blob/14a9350341846b9b1d21bb01dfcbcee593d30896/src/gmt_api.c#L9389

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.

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[weiji14]

Looks good, just a tiny suggestion.

[weiji14]

In #2398 and #2729, you have the GMT_DATASET and GMT_GRID classes in one file, but here, you've separated them into two files. I think this is ok, since we'll need to wrap GMT_IMAGE and GMT_CUBE at some point.