Cylindrical packed bed

[blaisb]

Description

The steady-state flow withing a cylindrical packed bed example was working fine, but the documentation was outdated. Furthermore, the figures we atrociously bad and were not representative enough. I have updated the example results and I have added new figures which illustrate the behavior of the VANS equation in a better fashion. I have also added a simple python post-processing script to do these calculations.

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

[lpsaavedra]

Minor comments/questions from my side, but seems good already