Correctly fill VariableFluxPack for edge fluxes in 2D

[acreyes]

PR Summary

• X3 flux is filled in 2D for Metadata::Faces with Metadata::WithFluxes.

fixes #1082

PR Checklist

• [x] Code passes cpplint
• [ ] New features are documented.
• [ ] Adds a test for any bugs fixed. Adds tests for new features.
• [x] Code is formatted
• [x] Changes are summarized in CHANGELOG.md
• [ ] Change is breaking (API, behavior, ...)
  • [ ] Change is additionally added to CHANGELOG.md in the breaking section
  • [ ] PR is marked as breaking
  • [ ] Short summary API changes at the top of the PR (plus optionally with an automated update/fix script)
• [ ] CI has been triggered on Darwin for performance regression tests.
• [ ] Docs build
• [ ] (@lanl.gov employees) Update copyright on changed files

[Yurlungur]

Ping @lroberts36 for second review.

[Yurlungur]

Not sure why this isn't automatically merging. Will force merge.