update retrieve_data in era5.py

[TimFuermann]

• changed download_format to .zip
• added entcrypting of .zip file from download and merging of contained .nc files
• add url

Closes # (if applicable).

Changes proposed in this Pull Request

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.

[TimFuermann]

This pull request should solve the issue mentioned in #413

[lkstrp]

Next to the comments, please add a release note. Afterwards we could merge this. Thank you !