Geologic Unit Dataset

**Description of the desired feature**
I would like a geologic unit dataset to be added to pygmt.datasets. After receiving help on the forums for [plotting a geologic map](https://forum.generic-mapping-tools.org/t/pygmt-coloring-polygons-from-a-converted-kml-file/1798), I'd like to contribute what I learned to the example gallery for how to use fig.plot to color polygons by the values in a column. However the PyGMT Contributors Guide says that examples should use data from pygmt.datasets.

**Are you willing to help implement and maintain this feature?** Yes/No
I am willing to contribute whatever I'm able to bringing this about and maintaining it across new PyGMT releases, though I admit my coding experience is limited.