Add RollingDataset to close past forcing data

[evetion]

Issue addressed

Fixes #1037

Explanation

Wflow uses deferopen=false in opening multifile forcing data, so every monthly file remains open. Each accessed forcing variable in each file gets a 64 MB HDF5 chunk cache.
For 10.5 years, 126 monthly files * 3 variables * 64 MB that results in roughly 24 GiB.

This adds a RollingDataset instead of the MFD NCDataset, that only holds one forcing file open at a time.

Checklist

• Updated tests or added new tests
• Branch is up to date with main
• Tests & prek hooks pass
• Updated documentation if needed
• Updated changelog.qmd if needed
• A review by Copilot was done to ensure the requirements in AGENTS.md are satisfied

Additional Notes (optional)

Changing deferopen=true makes the model slower, as it would reopen the file on each var access. Ideally we upstream this to NCDatasets, I will make an issue.

Partially AI generated with GPT-5.6 Sol.

[evetion]

Happy to have someone from the core team take over this PR :)