Tidier git history using squash merge

[weiji14]

Awesome to see so many new contributors joining icepyx! I'm sure we'll all get some great science done, but if I could make one small request - which is to keep the git history nice looking going forward. This is what the graph looks like at the moment:

It's fine to do merge commits when it's just one or two people working on a repository, but bigger projects with tens if not hundreds of contributors use squash merge commits. This keeps the git history linear looking, and we don't have big diffs from every single commit (especially important since jupyter notebooks can run into several MBs).

[JessicaS11]

Thanks @weiji14 for this suggestion. I've updated the settings to require a linear git history via squash merge commits to the development branch to which PRs are submitted.