New example: tracer in static mixer

The tracer through a CAD-defined junction was outdated, and an example using the tracer physics with the checkpoint/restart mechanism of Lethe was required for some time. In this PR, the former example is completely removed and replaced by this new example.

Description

The new example contains two simulations working in pair: an initial simulation to reach a pseudo steady state for the flow dynamics, and a final simulation with only a passive tracer transported through the domain.

Miscellaneous (will be removed when merged)

Currently, the animation shown at the end is hosted on my personal youtube channel. When we agree to a definitive version, this new version will be hosted on the Lethe-CFD youtube channel.

Additionally, this PR should be merged after #1343 is reviewed and accepted as it relies on changing the time step between simulations.

Checklist (will be removed when merged)

See this page for more information about the pull request process.

Code related list:

[x] Lethe documentation is up to date
[x] Copyright headers are present and 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] Links are added to parent .rst files
[x] The example is following the standard format

Pull request related list:

[x] 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

chaos-polymtl / lethe