code: Fix cdsapi syntax for ERA5

[euronion]

Supersedes #414 .

I closed #413 without taking notice of the PR #414 .

This PR implements the small changes to the API call proposed in the other PR, without the retrieval as e.g. zip file.

Thanks @TimFuermann for the contribution in the other PR!

Flagging @fneum and @lkstrp as they were involved in the review earlier.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.