[GH-2398] Implement geopandas union

[yunchipang]

Did you read the Contributor Guide?

• Yes, I have read the Contributor Rules and Contributor Development Guide

Is this PR related to a ticket?

• Yes, and the PR name follows the format [GH-XXX] my subject. Closes Geopandas: Implement union #2398

What changes were proposed in this PR?

Implements union

How was this patch tested?

Tests added

Did this PR include necessary documentation updates?

• Yes, I have updated the documentation.

[petern48]

Thanks again @yunchipang! No extra feedback on this PR

[petern48]

One thing unrelated to the code changes itself. I would prefer for you to continue linking the GitHub issue (#2398) in the PR description. I don't personally care for anything else in there (maybe Jia will disagree), but linking the issue is useful because GitHub has a feature for auto-closing the issue when the PR is merged, when we link it. More importantly, if someone else stumbles across the issue, we want them to see your linked PR, so they don't try to duplicate your work before yours is merged.

extra nit: I also prefer that we mention Geopandas in the PR title because the title eventually becomes the commit message after merging. (don't worry about your previous PRs). It's more descriptive in cases where anyone looks back through commit history or PR history (for debugging, for example). e.g. something like "Geopandas: Implement union" or "Implement geopandas union"

[yunchipang]

@petern48 yep sure will do. i did forget to tag issue number in this PR (sorry!) but the previous ones are all tagged. will make sure geopandas is mentioned in PR title moving forward as well. thanks for the heads up!