Total Field / Scattered Field Injection

[Antoniahuber]

📝 Pull Request Summary

• The beam can't pass the CPML because it is creating strong parasitic fields there disturbing the simulation
• The beam is only injected in the physical domain itself
• To avoid the divergence errors occurring from the beam being suddenly accelerated/stopped, the injected field at the boundary is being calculated and added/ subtracted at the boundary

🔧 Changes Made

-New injection scheme "transmissionline", only possible in combination with CPML and beta=1

✅ Checklist

• Code follows the project's style and guidelines
• Added tests for new functionality
• Updated documentation (mentioned in docs/ or included in examples/ and notebooks/)
• Verified that changes do not break existing functionality (automatic tests passing)

📌 Related Issues / PRs

Closes #issue-number