Mirror to code.usgs.gov

We are required to make USGS software available via a .gov URL.

See:
* Project mirrors API: https://code.usgs.gov/help/api/remote_mirrors.md
* Push mirrors: https://code.usgs.gov/help/user/project/repository/mirror/push.md#set-up-a-push-mirror-from-gitlab-to-github
* Pull mirrors: https://code.usgs.gov/help/user/project/repository/mirror/pull.md

Assuming it's technically feasible given our account settings, a pull mirror is sounding good to me, to maintain public access to and interactivity with the GitHub version of this repo.