Convert booleans arguments in build_arg_string, not in kwargs_to_strings

[seisman]

Description of proposed changes

See #640 and #639 (the discussions and the fix) for context.

Changes in this PR:

• 06bee27: Remove convert_bools parameter from kwargs_to_strings. So kwargs_to_strings won't convert boolean arguments.
• b7210ff: Handle boolean arguments in the build_arg_string function.
• 59a2b28: The boolean use_srtm parameter of load_earth_relief won't be converted to an empty string or removed anymore. Now it's safe to check if use_strm.
• a9d167d: The autolabel parameter of subplot function can be either bool or str, but we had to check if autolabel is not None. It's not intuitive. Actually, we made a mistake in the initial version (4126c16) and then fixed it (eadb847).
• aac61c2: Similarly, justify, angle, and font can be boolean, but we have to check if it's None. This commit also fixes the weird codes.

Fixes #640.

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• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

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[weiji14]

Just some minor comments for now. I think there's some room for improvement but will suggest more later (possibly after the weekend).

[weiji14]

A few suggestions, mostly on refactoring the if-then blocks. Feel free to disagree if it feels less readable to you.

[weiji14]

Just thinking ahead to resolve #483 (allowing list inputs to angle/list/justify), we could simply use if angle (returns True for list/int/float/str inputs) like so:

Suggested change

	if angle is True:
	kwargs["F"] += "+a"
	elif isinstance(angle, (int, float, str)):
	kwargs["F"] += f"+a{str(angle)}"
	if angle:
	kwargs["F"] += "+a"
	if isinstance(angle, (int, float, str)):
	kwargs["F"] += str(angle)

[seisman]

Actually, I was planing to work on #483, but I realized that the Figure.text() function needs more refactoring, so I opened PR #1121.

Personally, I prefer to make the changes to text.py as small as possible, and will address #483 after #1121 is merged.

[weiji14]

Ok, as you wish then.

[weiji14]

Suggested change

	if font is True:
	kwargs["F"] += "+f"
	elif isinstance(font, str):
	kwargs["F"] += f"+f{font}"
	if font:
	kwargs["F"] += "+f"
	if isinstance(font, str):
	kwargs["F"] += font

[weiji14]

Suggested change

	if justify is True:
	kwargs["F"] += "+j"
	elif isinstance(justify, str):
	kwargs["F"] += f"+j{justify}"
	if justify:
	kwargs["F"] += "+j"
	if isinstance(justify, str):
	kwargs["F"] += justify

[weiji14]

Thanks @seisman, good to finally close #640, and this change will open up a lot of new possibilities with PyGMT functions!