Change of main branch : from `master` to `main`

[YanNoun]

Dear users and contributors,

As per our contributing guidelines and code-of-conduct, we're moving away from using the terminology master to using main instead, when speaking of the default git branch.

Anyone who has checked out this repository locally will need to run the following to keep sync with master/main :

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

The OpenSfM team.