Usability: Native `Digital Rocks Portal` Data Fetcher

[jameslehoux]

Labels: usability, documentation, phase:1-battery
Priority: High (Quick Win for Onboarding)

Description

To ensure our tutorials are instantly reproducible, users need access to real, high-resolution tomographic datasets without bloating our git repository. The Digital Rocks Portal (DRP) hosts standard benchmark datasets for porous media, or alternatives.

We will implement a dataset fetcher that streams/downloads standard reference datasets directly into the user's local cache for immediate use in tutorials.

Acceptance Criteria

• Implement openimpala.datasets.fetch_example(name="berea_sandstone") using pooch or standard urllib to download and cache standard TIFF/RAW files.
• Ensure the downloaded datasets automatically instantiate into VoxelImage objects.
• Update all Jupyter notebook tutorials to use this fetcher, removing all hardcoded local file paths.