Add tutorial for contour maps

[willschlitzer]

Created a tutorial on the usage of the grdcontour method to demonstrate creating a plot with contour lines and adjusting the intervals, annotations, and limits. Per the feedback from @seisman, I'm resubmitting this under it's own branch from my fork and not including the .ipynb file.

[willschlitzer]

@seisman Pardon my unfamiliarity with the open source contributing process, but I just want to confirm that you are not waiting on any changes/additional contributions for this PR? Thanks!

[seisman]

@willschlitzer Sorry for the absence. I'm a little busy yesterday and will leave more reviews/comments in the next few days.

[seisman]

@willschlitzer One more thing, although it's not mandatory, it would be better if you can wrap the lines to no longer than 88 characters, to make it more readable.

[willschlitzer]

@seisman Now that I've received feedback on how to document the example code, I decided to commit the example I had to include a colormap on the contour map. It seemed easier to commit it now to include it in this pull request, but please let me know if I overstepped and should have instead waited for a separate pull request.

[seisman]

@seisman Now that I've received feedback on how to document the example code, I decided to commit the example I had to include a colormap on the contour map. It seemed easier to commit it now to include it in this pull request, but please let me know if I overstepped and should have instead waited for a separate pull request.

It looks good to add a color map.

[seisman]

@willschlitzer I just leave more comments on your new example. I think the tutorial is pretty good now.

The only issue is that the tutorial is using the "15s" earth relief data. As you may know or not, these earth relief data are stored in the GMT data server, and GMT (or PyGMT) will download the grid tiles for the first time the data is used. Usually, each tile is smaller than 10 Mb, so it's not a problem for users. However, the Linux and Windows agents of GitHub Actions (the machine/service we're using to run tests and build documentation) sometimes can't download data from the GMT data server (#434 (comment)), so we have to cache the data using a macOS agent and let Linux/Windows re-use the caches (#530).

Currently, only the following earth relief data are cached:

pygmt/.github/workflows/cache_data.yaml

Lines 33 to 36 in 5c8b851

	gmt which -Ga @earth_relief_10m_p @earth_relief_10m_g \
	@earth_relief_30m_p @earth_relief_30m_g \
	@earth_relief_01d_p @earth_relief_01d_g \
	@earth_relief_05m_g

It would be good if you can modify your examples using a low-resolution grid (05m or 10m, you can increase the region setting), otherwise, we have to cache the 15s earth relief data, too.

[willschlitzer]

@seisman I updated the region setting to "05m". I'm pretty unfamiliar with using the GMT Earth relief data date, but I was unable to use "10m," as I received a NotImplementedError that said it isn't supported for that resolution.

Thanks for all of your feedback (and patience with my mistakes)!

[seisman]

Please also click the "Update branch" button to merge the latest changes in the master branch to your branch.

[seisman]

Now it looks pretty good to me.

[seisman]

I'll merge this PR tomorrow. @GenericMappingTools/python if you find anything that should be fixed, please leave your comments in the next 24 hours.

[willschlitzer]

@seisman Thanks for all of the feedback throughout this process. I look forward to using my lessons learned on future pull requests on this project.

[seisman]

@willschlitzer As mentioned in #707 (comment), feel free to open a new pull request to add yourself to the AUTHORS.md file.