Figure.show: Allow keyword arguments passed to Figure.psconvert

[seisman]

Description of proposed changes

Add **kwargs to Figure.show so that it can pass any keyword arguments to Figure.psconvert.

Figure.savefig() already have **kwargs.

Fixes #2038

Reminders

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• 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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[seisman]

Unsure if we should use kwargs or **kwargs here.

[weiji14]

I had a look at some projects and there seems to be a mix 😅. Let's go with **kwargs following https://docs.xarray.dev/en/v2022.06.0/generated/xarray.open_dataset.html, unless anyone else has a good reason on why not.

[weiji14]

I find it a bit confusing that dpi (an option in psconvert) was previously an extra option or kwarg, but now there's both dpi and **kwargs in the Figure.savefig(...) method shown in the documentation:
.

Not sure what's the best solution here. Removing the dpi parameter documentation doesn't seem ideal, nor does including all of psconvert's parameters in savefig. Would it make sense then to put the dpi documentation under **kwargs? Or is there a cleaner solution?

[seisman]

I agree. Actually some of psconvert's parameters (e.g., prefix, fmt) can NOT be used in Figure.show or Figure.savefig.

Perhaps we should say:

**kwargs: dict
    Additional keyword arguments passed to `pygmt.Figure.pscovert`. 
    Valid parameters are `gs_path`, `gs_option`, `resize`, `bb_style`, and `verbose`.

[weiji14]

Yes! Also need to do a similar thing for the fig.show docstring.