fenicsx: Flux BC support

[jhdark]

Proposed changes

With this change, users will be able to impose flux boundary conditions:

$-D\nabla c_{\mathrm{m}} \cdot \mathbf{n} = f(x, y, z, t) \ \mathrm{on} \ \partial\Omega$

Users can implement this like:

my_model.boundary_conditions = [
        F.FluxBC(subdomain=right, value=10, species=H),
]

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)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.51%. Comparing base (916c11c) to head (89e6de0).

❗ Current head 89e6de0 differs from pull request most recent head 044ea98. Consider uploading reports for the commit 044ea98 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx     #737      +/-   ##
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+ Coverage    98.39%   98.51%   +0.12%     
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  Files           27       28       +1     
  Lines         1434     1554     +120     
===========================================
+ Hits          1411     1531     +120     
  Misses          23       23              

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[RemDelaporteMathurin]

Isn't this a bit dangerous? Say I check if it's time-dependent before assigning it a value by mistake, I may think it's not time-dpeendent but then it becomes time dependent if the value is say a function of time.

Should we raise an error instead?

[jhdark]

Yeah this is valid, but would be worth changing the other cases of this property in other classes too

[RemDelaporteMathurin]

Opened an issue to track progress on this #744

[RemDelaporteMathurin]

Is this needed? I would replace by if self.time_dependent? what if it isn't? should we raise an error? or just a print?

[jhdark]

This is how we also handle it in sources, and Dirichlet bc, so maybe we could change this for all of them in another PR?

[RemDelaporteMathurin]

Sounds good with me

[RemDelaporteMathurin]

I opened an issue #743 to remind us of this

[RemDelaporteMathurin]

@jhdark just a couple of documentation changes and then I think we are good to go with this one. We will later add concentration dependent (Robin) ParticleFluxBC