Use "# %%" as code block separators in gallery examples

[seisman]

For gallery examples, it usually makes sense to split a long script into multiple blocks using block separators. The Sphinx-Gallery package supports three different "block separators/splitters"

1. more than 20 # characters, i.e., ###################################################
2. # %%
3. #%%

Reference: https://sphinx-gallery.github.io/stable/syntax.html#embed-rest-in-your-example-python-files

The separator #%% will always be formatted to # %% by black, following PEP-8 convention, so this kind of separator can be ignored.

Currently, we use the # separator in our gallery examples (e.g., https://github.com/GenericMappingTools/pygmt/blob/main/examples/gallery/maps/tilemaps.py). I propose to use # %% instead.

The main reason is that # %% is also supported by Visual Studio Code and other editors/IDEs (see https://code.visualstudio.com/docs/python/jupyter-support-py#_jupyter-code-cells), making Python scripts working like Jupyter Notebooks. The # %% also can save us a lot of keystrokes.

[weiji14]

Agree to use this 'percent' format, which was introduced by Spyder in 2013 according to https://jupytext.readthedocs.io/en/latest/formats-scripts.html#the-percent-format. For reference, the 3 different types of separators for code blocks in Sphinx Gallery is documented at https://sphinx-gallery.github.io/stable/tutorials/plot_parse.html#alternating-text-and-code.