Improve granular-dam-break

[blaisb]

Description

Very small changes. I increased the amount of load balancing for performance reason, but the example is pretty simple and efficient.

Miscellaneous (will be removed when merged)

Checklist (will be removed when merged)

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Code related list:

• [x] All in-code documentation related to this PR is up to date (Doxygen format)
• [x] Copyright headers are present and up to date
• [x] Lethe documentation is up to date
• [x] The branch is rebased onto master
• [x] Code is indented with indent-all and .prm files (examples and tests) with prm-indent
• [x] If parameters are modified, the tests and the documentation of examples are up to date
• [x] Changelog (CHANGELOG.md) is up to date if the refactor affects the user experience or the codebase

Pull request related list:

• [x] No other PR is open related to this refactoring
• [x] Labels are applied
• [ ] There are at least 2 reviewers (or 1 if small feature) excluding the responsible for the merge
• [x] If this PR closes an issue or is related to a project, it is linked in the "Projects" or "Development" section
• [x] If any future works is planed, an issue is opened
• [x] The PR description is cleaned and ready for merge