POC: Add Figure.choropleth to plot choropleth maps

[seisman]

Description of proposed changes

This PR adds the Figure.choropleth method to plot choropleth maps.

Implementations in other packages

• GeoPandas:
  • https://geopandas.org/en/stable/docs/user_guide/mapping.html
  • https://geopandas.org/en/stable/docs/reference/api/geopandas.GeoDataFrame.plot.html
• GeoPlot:
  • https://residentmario.github.io/geoplot/plot_references/plot_reference.html#choropleth
• Plotly:
  • https://plotly.com/python/choropleth-maps/
  • https://plotly.com/python-api-reference/generated/plotly.express.choropleth

Implementation in PyGMT

In PyGMT, Figure.choropleth can be implemented by wrapping the Figure.plot method (see the changes for details). It's API is designed like:

Figure.choropleth(data, fill, **kwargs):

in which,

• data can be :class:geopandas.GeoDataFrame, geo-like objects with __geo_interface or a OGR_GMT file.
• fillcol is the name of the column whose values will be used to assign the colors
• **kwargs are other parameters that can be passed to Figure.plot. Useful parameters are like cmap, pen, transparency and more.

Example

import geopandas as gpd
import pygmt

gdf = gpd.read_file("https://geodacenter.github.io/data-and-lab/data/airbnb.zip")

fig = pygmt.Figure()
pygmt.makecpt(
    cmap="acton",
    series=[gdf["population"].min(), gdf["population"].max(), 10],
    continuous=True,
    reverse=True,
)
fig.choropleth(gdf, fillcol="population", pen="0.3p,gray10")
fig.colorbar(frame=True)
fig.show()

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 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.
• Use underscores (not hyphens) in names of Python files and directories.

[michaelgrund]

Really like this idea, makes it much easier to generate a simple choropleth map. Will wait to continue working on #2796 until this PR is merged.

[michaelgrund]

Suggested change

	fillcol : str
	column : str

I feel fillcol is not the correct name for this parameter. Would simply use column, as it is also used in e.g. geopandas.