Examples update for concentric-heat-exchanger and melting-cavity

[OresteMarquis]

Description

Update of the .prm and .rst files of the concentric-heat-exchanger and melting-cavity examples for the Lethe 1.0 release. The melting-cavity gave different results for the viscous penalization so I updated the figures in the documentation.

Miscellaneous (will be removed when merged)

Checklist (will be removed when merged)

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Code related list:

• Lethe documentation is up to date
• Copyright headers are present and up to date
• The branch is rebased onto master
• Code is indented with indent-all and .prm files (examples and tests) with prm-indent
• Links are added to parent .rst files
• The example is following the standard format

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 any future works is planned, an issue is opened
• The PR description is cleaned and ready for merge