Correct units for cylindrical and spherical fluxes

[RemDelaporteMathurin]

Proposed changes

Fix for #828

We should add this units property to all derived quantities!

Types of changes

What types of changes does your code introduce to FESTIM?

• [x] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Code refactoring
• [ ] Documentation Update (if none of the other choices apply)
• [ ] New tests

Checklist

• [x] Black formatted
• [x] Unit tests pass locally with my changes
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.56%. Comparing base (10bfae9) to head (4e45b4a). Report is 68 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #835 +/- ## ======================================= Coverage 99.56% 99.56% ======================================= Files 61 61 Lines 2747 2754 +7 ======================================= + Hits 2735 2742 +7 Misses 12 12 ```

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[RemDelaporteMathurin]

Could take the opportunity to add the attributes to the child classes, but not necessary

What child classes?

[jhdark]

Could take the opportunity to add the attributes to the child classes, but not necessary

What child classes?

SurfaceFluxCylindrical and SurfaceFluxSpherical

[RemDelaporteMathurin]

Could take the opportunity to add the attributes to the child classes, but not necessary

What child classes?

SurfaceFluxCylindrical and SurfaceFluxSpherical

Well that's already what this PR is doing 😕