Refactor `create_value_fenics` for source and fluxes

[RemDelaporteMathurin]

Proposed changes

Based on a discussion I had with @jhdark we realised that we only needed to interpolate values onto a Function for dirichletBCs (and maybe InitialConditions). For fluxes and sources, since those are added into the formulation we can just put space-time dependent expressions as a ufl Expression, moreover it doesn't need to be updated in this case.

This is a first step towards #749

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)
• [x] 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
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] 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 98.42%. Comparing base (aac14ac) to head (6e1cf21). Report is 10 commits behind head on fenicsx.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## fenicsx #750 +/- ## =========================================== - Coverage 98.49% 98.42% -0.08% =========================================== Files 28 28 Lines 1532 1519 -13 =========================================== - Hits 1509 1495 -14 - Misses 23 24 +1 ```

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