Fix bdf extrapolation bug

[AmishgaAlphonius]

Description

A bug in the VOF auxiliary physics was discovered when rerunning the gravity wave example. In the PR #994, BDF extrapolation of the velocity vector field in the VOF auxiliary physic was moved to the scratch data. However, the implementation was bypassed by an "if" condition.

Solution

This bug is fixed with this PR, and an application test was added to ensure that the feature remains intact with future implementations.

Testing

The new application test: applications_tests/lethe-fluid/vof-velocity-extrapolation.prm

Miscellaneous (will be removed when merged)

A deal.ii version dependency when using kelly mesh refinement on the phase fraction with the BDF velocity extrapolation in VOF has been noticed, an issue will be opened to investigate this further.

Checklist (will be removed when merged)

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Code related list:

• All in-code documentation related to this PR is up to date (Doxygen format)
• Lethe documentation is up to date
• Fix has unit test(s) (preferred) or application test(s), and restart files are in the generator folder
• The branch is rebased onto master
• Changelog (CHANGELOG.md) is up to date
• Code is indented with indent-all and .prm files (examples and tests) with prm-indent

Pull request related list:

• Labels are applied
• There are at least 2 reviewers (or 1 if small feature) excluding the responsible for the merge
• If this PR closes an issue or is related to a project, it is linked in the "Projects" or "Development" section
• If the fix is temporary, an issue is opened
• The PR description is cleaned and ready for merge