Changing default branch name to main

[VinzenzBildstein]

To keep in line with the GRSISort and GRSIData repositories I propose that we change the name of the default branch from "master" to "main" (this is the new default name for branches on GitHub).

After changing the name of the branch in the repository it will be necessary to change the local copies as well (I think forks will reflect the change automatically?) using these commands:

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

Unless there are any objections to this I will do the change on Sunday, March 27th.