Renaming 'master' to 'main'

[David Hoese]

Hi everyone,

I'm hoping to start work on this project again. I've been renaming a lot of my projects to use a "main" branch instead of "master" and decided to do the same for geoxarray. The term is offensive (needlessly as other branch names would do) and at this point it is inconsistent with a large number of projects and future github projects (which now default to main). If you have an issue with this change please feel free to contact me directly. I'm just making this issue to publicly say "yes, I did it and I did it on purpose".

Now let's see what I can turn this project into...