Switch default cutout compression to zstd with BitRound quantization - #529

Closes #508.

Changes proposed in this Pull Request

Cutout.prepare now compresses cutouts with zstd (complevel 3) after BitRound quantization to 14 mantissa bits (Klöwer et al., 2021). The maximum relative error is 2**-15 = 3.1e-5.

• Default: {"compression": "zstd", "complevel": 3, "quantize_mode": "BitRound", "significant_digits": 14}. For the old behaviour, pass {"compression": "zlib", "complevel": 9, "shuffle": True}.
• All variables: the encoding now applies to all variables of the cutout, not only to the new ones. xarray < 2026.9 drops the zstd setting of existing variables when it rewrites the file. Without this change, a second prepare call would store the earlier variables uncompressed.
• Dependency: requires netCDF4>=1.7.4, whose wheels bundle the zstd plugin. conda-forge libnetcdf (tested with 4.10.1) also works.

Benchmark

Europe, January to March 2013, 2160×131×177, 3.2 GB in memory:

Compatibility

• Existing zlib cutouts still read without changes. A new prepare call recompresses them with zstd.
• netCDF4<1.7.4 (pip) cannot read the new cutouts. It fails with NetCDF: Filter error: undefined filter encountered.

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.