Cleaning up configuration histories

[dougiesquire]

I was chatting with @micaeljtoliveira this morning and he made a point that I think we should seriously consider.

We've been having a lot of difficulty using the automated cherry-picking. That's in large part due to the fact that the histories of our configuration branches have become unnecessarily complex and divergent. We should consider rewriting the history of these branches to bring them in line and make development easier going forward. As a first step, this would require reconciling the unwanted differences between the branches (see @aekiss' neat comparison table in the access-om3-configs README). Then, in the simplest case we could just squash all the history on each branch into a single commit, though it would be better if we could split commits into things that are shared across branches and things that aren't.