Rename master to main

[ocefpaf]

@efiring and @DocOtak what do you think? It should not be too disruptive. Most of the projects I'm part of already moved away from the name master.

[DocOtak]

I've not taken an active role in this type of decision and will be happy with whatever is decided (or what my tools default to).

[efiring]

For the sake of adhering to the standard set by the git default--basically, for consistency--I'm OK with the change if you would like to execute it.

[ocefpaf]

Done!

[DocOtak]

I see a bunch of refs to "master" in the gh actions, do those need to be updated?

[ocefpaf]

I see a bunch of refs to "master" in the gh actions, do those need to be updated?

See #105