allow Value for SourceBase

[RemDelaporteMathurin]

Proposed changes

We noticed with @jhdark that ParticleSource didn't accept Value as value argument. We had to make a custom class here

This PR simply accepts Value objects.

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

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.88%. Comparing base (db928e5) to head (c49b110).
Report is 114 commits behind head on fenicsx.

Files with missing lines	Patch %	Lines
src/festim/source.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx     #979      +/-   ##
===========================================
- Coverage    95.91%   95.88%   -0.04%     
===========================================
  Files           46       46              
  Lines         2719     2721       +2     
===========================================
+ Hits          2608     2609       +1     
- Misses         111      112       +1     

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