Rename "master" branch as "main"

[desruisseaux]

This is a proposal to rename the "master" branch as "main". The procedure is described here and is quite simple (GitHub takes care of creating redirection links).

[desruisseaux]

Hello @keilw. It seems that I do not have administrative right for this task. If okay for you, could you go on https://github.com/unitsofmeasurement/seshat/branches click on the pencil on the right side of "master" and rename as "main" please?