toggle for (concurrent) monthly ERA5 requests; progressbar disabled by default

[fneum]

Closes #371.

Changes proposed in this Pull Request

Adds option to toggle whether ERA5 downloads are requested in monthly or annual chunks with option to place concurrent requests with keyword argument cutout.prepare(monthly_requests=False, concurrent_requests=False).

Potential performance improvements and functionality tests shown in #371.

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.

[fneum]

See performance checks in #371

[FabianHofmann]

This is great. Would you mind putting a test there for checking both functionalities? Otherwise feel free to merge

[fneum]

I have tested this extensively and it works well!