feat: add thermic source

[pluAtAnsys]

Description

Add thermic source in core layer inside the core.source module
Follow the existing architectural structure, using current source implementations as reference.
Ensure default values are consistent with SCDM Speos standards.

Documentation
Provide a usage example in the core module’s documentation, demonstrating how to initialize and use the new ambient source.

Testing
Create a unit test for the thermic source functionality within the core module to ensure stability and correctness.

Issue linked

Add Thermic source in core layer #540

Checklist

• I have tested my changes locally.
• I have added necessary documentation or updated existing documentation.
• I have followed the coding style guidelines of this project.
• I have added appropriate tests (unit, integration, system).
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved by the PR if any.
• I have assigned this PR to myself.
• I have made sure that the title of my PR follows Conventional commits style (e.g. feat: add optical property)
• I have agreed with the Contributor License Agreement (CLA).

[codecov]

Codecov Report

❌ Patch coverage is 76.47059% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.65%. Comparing base (a37d893) to head (4e60a56).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ansys/speos/core/source.py	78.12%	14 Missing ⚠️
src/ansys/speos/core/project.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #651      +/-   ##
==========================================
- Coverage   88.80%   88.65%   -0.15%     
==========================================
  Files          39       39              
  Lines        5634     5702      +68     
==========================================
+ Hits         5003     5055      +52     
- Misses        631      647      +16     

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[pluAtAnsys]

please consider refactor this into unittest or example.
if needed, add the speos file used into the tests/assert folder