Optimize NS-VOF assembler

[hepap]

Description

The NS-VOF assembler was not optimized as the NS assembler. The if conditions on the dof components were added to not compute multiplications that are a priori 0.

Testing

None of the application tests have changed. The computational time gain is about 20% by iteration on the 3D lpbf benchmark case for a simulation of about 300 iterations. I can do it for a complete simulation if you thing it is worth doing it :)

Documentation

No new documentation needed.

Miscellaneous (will be removed when merged)

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
• New feature 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

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• If the fix is temporary, an issue is opened
• The PR description is cleaned and ready for merge