Fix utils.get_axis with kwargs

[headtr1ck]

• Closes cannot set figsize when plotting map with cartopy #7078
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[DWesl]

I think the current default for two-dimensional plots is to try to re-use an existing axis if neither row nor col are set (implied by documentation for ax argument in the description of the DataArray.plot descriptor). Would this PR change that behavior, and should that be documented?

[headtr1ck]

I think the current default for two-dimensional plots is to try to re-use an existing axis if neither row nor col are set (implied by documentation for ax argument in the description of the DataArray.plot descriptor). Would this PR change that behavior, and should that be documented?

If you do not specify size, figsize, aspect, row, col or ax, it should still use the current axis.
But you are right, that deserves a test :)

[headtr1ck]

And additionaly, we should change the description. The one of DataArray.plot.line is better.
I will change this.

Thanks for noticing!