Refactor SIPNET to separate version logic from availability checks

[Thomas-Sedhom]

• Centralize SIPNET version capabilities in write.configs.SIPNET.
• Separate version gating from input/output availability checks in model2netcdf.SIPNET.

Description

This keeps version driven behavior in one place and avoids interleaving it with presence checks for optional inputs or output columns, improving readability and maintainability.

Motivation and Context

Solve: #3854

Review Time Estimate

• Immediately
• Within one week
• When possible

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• My name is in the list of CITATION.cff
• I agree that PEcAn Project may distribute my contribution under any or all of
  • the same license as the existing code,
  • and/or the BSD 3-clause license.
• I have updated the CHANGELOG.md.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[infotroph]

@Thomas-Sedhom Thanks for opening this and my apologies that it waited so long for review.

1. Are you interested in continuing work on this? If so I'm happy to advise more (and faster!), if not I'll push the updates needed to get it merged; either way you'll have author credit on the changes to date.
2. In the time this was waiting, model2netcdf.SIPNET has changed to ignore the revision string entirely and clarify that behavior depends only on the column names in sipnet.out, so I'm leaning toward dropping all model2netcdf changes from this PR to focus it on write.config. Does that make sense to you?