[fix] Fix for dlwp_healpix dataloader not subsetting channels correctly

[daviddpruitt]

Modulus Pull Request

Description

Fixes a bug where the dlwp_healpix dataloader uses a prebuilt dataset with more channels than used by the model. This fix adds channel selection so only the necessary channels are loaded.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.
• [x] The CHANGELOG.md is up to date with these changes.
• [x] An issue is linked to this pull request.