Add support for errorbars in scatter plots

[dschwoerer]

• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

I have added the possibility to add yerr and/or xerr to Dataset.plot.scatter.

I know that this needs tests and so on, but I wanted to know whether this is of general interest?

I have found #2264 for dataarrays, which wasn't merged, and one of the issues was that the return type changed, as is here the case.