Handle setting special parameters without default settings for config

[weiji14]

Description of proposed changes

There are 8 parameters in gmt.conf that don't have default settings, meaning they can't be reverted back easily when using pygmt.config. They are:

• FONT (which changes the FONT_* settings below except FONT_LOGO)
  • FONT_ANNOT_*
  • FONT_LABEL
  • FONT_TITLE
  • FONT_HEADING (requires Wrap subplot #412)
  • FONT_TAG (requires Wrap subplot #412)
• FONT_ANNOT
• FORMAT_TIME_MAP*
• MAP_ANNOT_OFFSET
• MAP_GRID_CROSS_SIZE
• MAP_GRID_PEN
• MAP_TICK_LENGTH
• MAP_TICK_PEN

We can manually set the relevant *_PRIMARY and *_SECONDARY parameters for 7 of them (excluding FONT) in an allowlist.

Fixes #409

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.

[weiji14]

Just need some extra tests to ensure that setting FONT changes FONT_HEADING and FONT_TAG when using subplots. Might need to complete #412 first 😉

[weiji14]

Just need some extra tests to ensure that setting FONT changes FONT_HEADING and FONT_TAG when using subplots. Might need to complete #412 first wink

Functionality-wise, this bugfix PR is done. We could add the tests for FONT_HEADING and FONT_TAG at a later date once the subplot wrapper is done. What do you think? cc @liamtoney

[seisman]

Looks good to me.