Add scaling for HPX dataloaders, code cleanup

[daviddpruitt]

Modulus Pull Request

Description

This adds scaling values for constant values in Healpix datasets.
If scaling values are given they will also be required for constant fields.
Removed duplicate code from coupled healpix dataloader to eliminate differing behaviour.

Checklist

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

Dependencies

[daviddpruitt]

/blossom-ci