Let Figure.savefig support filenames with spaces

[seisman]

Description of proposed changes

Figure.savefig() doesn't work if filenames contain spaces.

Here is an minimal example to reproduce the issue:

import pygmt
fig = pygmt.Figure()
fig.basemap(projection="X10c/10c", frame=True, region=[0, 10, 0, 10])
fig.savefig("map 2.png")

This PR fixes the issue by adding double quotes around filenames.

NOTE: Using filenames with spaces is NOT recommended.
However, we have to support it, because sometimes filenames of some
baseline images must contain spaces.
See https://github.com/GenericMappingTools/pygmt/runs/2179626599 for an example failure.

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

[weiji14]

NOTE: Using filenames with spaces is NOT recommended.
However, we have to support it, because sometimes filenames of some
baseline images must contain spaces.

See my comment at #804 (comment) on how to fix the space issue. If the tests pass, do we still need this PR?

[seisman]

Nice fix in #804 (comment).

I think this PR is still needed for people who want to use spaces. It's allowed in GMT (https://docs.generic-mapping-tools.org/dev/begin.html#examples).

[seisman]

@weiji14 Any other concerns?

[seisman]

Ping @GenericMappingTools/pygmt-contributors for comments if we should allow whitespace in figure names.

[core-man]

Ping @GenericMappingTools/pygmt-contributors for comments if we should allow whitespace in figure names.

I think so and this PR is simple enough.

[weiji14]

I'm still reluctant to allow spaces since it adds an if-clause to the code, but ok with supporting it if three or more people agree.

[willschlitzer]

Ping @GenericMappingTools/pygmt-contributors for comments if we should allow whitespace in figure names.

I think it is good to allow this. I'm glad there is the disclaimer that spaces in filenames isn't recommended, but better to increase compatibility.