rename "master" to "main"

[lheagy]

Similar to simpeg/simpeg#980, we will update the name of our main branch to "main" in our geosci repos

• geosci-labs
• gpg
• gpgLabs
• em
• website (including courses, disc, etc)
• toolkit

to get these changes locally, you will need to run

git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a

[lheagy]

👋 @yangdikun, I will let you update the

• ess302website repo

if you like. It is quite straightforward: https://github.com/github/renaming
Feel free to ping if you have any questions!