Linear solver for post-process chemical potential

[RemDelaporteMathurin]

Proposed changes

Sometimes, when using conservation of chemical potential, the non-linear solver in the post-processing step (from theta to c) doesn't converge.
To fix this, this PR replaces the Non linear solver in the post-processing step for the chemical potential by a linear solver.

Couldn't write a test for this as I couldn't reproduce a case where the solver diverges.

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 lines are covered by tests ✅

Comparison is base (e751955) 98.84% compared to head (62e4d72) 98.84%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #579      +/-   ##
==========================================
- Coverage   98.84%   98.84%   -0.01%     
==========================================
  Files          56       56              
  Lines        2073     2069       -4     
==========================================
- Hits         2049     2045       -4     
  Misses         24       24              

Files	Coverage Δ
festim/concentration/theta.py	100.00% <100.00%> (ø)

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[RemDelaporteMathurin]

@jhdark do you mind reviewing this?

[jhdark]

Looks good to me, much cleaner too, and more inline comments is helpful