Added add_cyclic to API reference and What's new Version 0.21

[mcuntz]

Rationale

I had added the convenience utility function add_cyclic in 2021. It was merged in Nov 5, 2021 by @greglucas.
We had apparently forgotten to add it to the API reference in the docs. It also did not make it into What's new Version 0.21 nor into the milestones on Github.
So I added util.add_cyclic in the transformations.rst in the documentation.
Because the PR 1753 was not included in the milestones of v0.21, I added it into the What's New v0.21.rst.

Implications

It will just be easier to find the functionality. Otherwise one could only find it in the gallery example.

Notes

Unrelated to this PR are two remarks:

1. One of the tests fails on my machine: lib/cartopy/tests/mpl/test_features.py::test_natural_earth The only differences between Expected and Actual images are some dashed country boundaries. I would recommend to make the coutry boundaries in solid lines to avoid this problem between matplotlib versions.
2. Building the documentation also fails on examples/miscellanea/tube_stations.py I have not investigated the problem. It actual fails in pyproj.

My system: macOS 13.4, Python 3.10.9 and in cartopy directory

python -m pip install --upgrade pyshp
python -m pip install "shapely<2" --no-binary shapely
python -m pip install cython numpy geos pyproj matplotlib owslib pillow scipy packaging pytest pytest-mpl pytest-xdist beautifulsoup4 pydata-sphinx-theme sphinx sphinx-gallery flake8 pre-commit pykdtree setuptools_scm fiona pep8
pip install -e .

[greglucas]

Thanks for adding this! Sorry for missing it in the last release notes. Just a minor suggestion to make it consistent with the above bullets if you're OK with adding your name.

[greglucas]

Suggested change

	* Adding a new convenience utility function :func:`~cartopy.util.add_cyclic` for
	* Matthias Cuntz added a new convenience utility function :func:`~cartopy.util.add_cyclic` for

[mcuntz]

@greglucas O.K. thanks.

[QuLogic]

Sorry, I don't quite understand what this is saying. The clause describing the utility function has several subclauses defining what it's useful for, but nothing really describing what it does.

[mcuntz]

I expanded on the statement about add_cyclic_point in v0.11.rst, which said:

• Andrew Dawson has added a new convenience utility function :func:~cartopy.util.add_cyclic_point to add a cyclic point to an array and optionally to a corresponding 1D coordinate.

I am happy for any suggestions in better English ;-)

How's about:

Matthias Cuntz added a new convenience utility function :func:~cartopy.util.add_cyclic. This is an extension of :func:~cartopy.util.add_cyclic_point for 2-dimensional coordinates lat and lon. (:pull:1753)

[mcuntz]

Sorry forgot to ping: @QuLogic @greglucas

[QuLogic]

That sounds good. No need to ping; we are subscribed automatically after commenting.