Warn about derived quantities in non cartesian

[RemDelaporteMathurin]

Proposed changes

This PR adds a warning when users employ SurfaceFlux with a non-cartesian mesh (eg. an axisymmetric 2D mesh).

Indeed, the flux (example here for particles) expression is

$$ J_\mathrm{total} = \int J \cdot n \ dS $$

where $n$ is the normal vector and

$$ J = - D \nabla c $$

In cartesian coordinates, the element $dS$ is $dS = dx \ dy $, assuming it is $y$ independent $dS = y \ dx$.
And this is the default behaviour in fenics.

However in cylindrical $(r, \theta, z)$, the surface element is $dS = r \ dr \ d\theta$, assuming it is axisymmetric ($\theta$ independent), then $dS =\theta \ r \ dr$

The flux becomes:

$$ J_\mathrm{total} = \int J \cdot n \ dS = \theta \int r \ J \cdot n \ dr $$

which is NOT the default behaviour of SurfaceFlux.
While we wait for a native support of fluxes in cylindrical coordinates to be implemented, users should at least be warn that this may be an issue.

The same goes for TotalVolume , AverageVolume , etc.

Thanks @WeaverC20 for helping spotting this

Types of changes

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• 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

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• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (49d18e8) 99.47% compared to head (33c4288) 99.47%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #688   +/-   ##
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  Coverage   99.47%   99.47%           
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  Files          58       58           
  Lines        2280     2286    +6     
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+ Hits         2268     2274    +6     
  Misses         12       12           

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[RemDelaporteMathurin]

@jhdark can we merge this before #689 ?

[jhdark]

Looks good to me!