Fix SurfaceFlux with Soret

[RemDelaporteMathurin]

Proposed changes

This is a fix for #830.

The compute method of SurfaceFlux had a soret argument defaulting to False. However, this was not integrated correctly with the rest of the implementation.

What I did is I removed this argument and replaced it by an attribute of SurfaceFlux. This attribute is set to True or False during Simulation.initialise. Also, the T attribute of SurfaceFlux is set at the same time in order to calculate the soret flux.

This bug did not affect any of the actual solving step and the equations were correctly solved with Soret. This is only a post-processing bug with SurfaceFlux (the other derived quantities are unaffected).

Note: I propagated this fix to the cylindrical and spherical versions of SurfaceFlux.

Types of changes

What types of changes does your code introduce to FESTIM?

• 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

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.55%. Comparing base (40cdd22) to head (fb360de).
Report is 80 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #831   +/-   ##
=======================================
  Coverage   99.55%   99.55%           
=======================================
  Files          61       61           
  Lines        2718     2724    +6     
=======================================
+ Hits         2706     2712    +6     
  Misses         12       12           

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[KulaginVladimir]

The fix seems ok.

I have a couple of questions below.

[KulaginVladimir]

I suppose everything is ok.