Add option to define custom format of units in plots

[headtr1ck]

• [x] Tests added
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst

We encountered the issue that we should plot units as (unit) instead of [unit]. This PR enables us to do exactly this, easier to change this at the source ;)

I think setting this as a global option is the correct approach, but feel free to propose alternatives :)

[dcherian]

yeah I'm not sure we should do this. Is there a more matplotlib-native way through pint perhaps?

[headtr1ck]

I have simplified this approach, now you define directly the format string instead of the brackets. Much simpler and more flexible.

If you decide to scrap this, also fine by me :)

[headtr1ck]

Is the current approach still too specific? I'm fine with closing this.

[max-sixty]

(no particularly strong view from me, I would generally avoid global options as much as possible. I'm not sure whether this is viable on .plot though...)

[dcherian]

I'm also inclined to avoid more global options. Our general suggestion in this case is to simply fix up the plot after. I also wonder if there's a way to control formatting by passing pint arrays through.