Update name of master branch

[ljhwang]

"master" branch needs to be renamed to "main"

[gassmoeller]

I have updated the default branch name to "main". You will likely be notified by github the next time you open this repo or your github fork. You will have to make the same change in your github fork under Settings->General->Default branch (e.g. https://github.com/geodynamics/best_practices/settings but your username instead of geodynamics).

To update local clones you need to run the following commands inside the local repository (make sure you are on master and you have updated your github fork if origin points to your github fork):

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