Update transient flow around cylinder example

The transient flow around a cylinder example works very well. However, some small things and a typo needed to be fixed.

A) Keeping the .msh file inside the folder is not a good idea I think. We have moved away from this. I have removed the file but kept the .geo file

B) I have added instructions on how to use the .geo file to make the mesh

C) The AMG preconditioner does not need additional parameters, the default values are fine.

D) There was a weird typo on the CL values, there was a zero too much there. I have fixed that.

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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

chaos-polymtl / lethe