Refactor simulation inputs to use pydantic

### 📝 Description of the feature

AdditiveMaterial includes a lot of code which is built in to the pydantic library. Dunder methods `__repr__` and `__eq__` should be removed.

### 💡 Steps for implementing the feature

_No response_

### 🔗 Useful links and references

https://docs.pydantic.dev/latest/
https://youtu.be/502XOB0u8OY?si=YGVcgp72C0NpW0iZ

Locate the AdditiveMaterial class and compare its behavior with the pydantic documentation linked in the issue. Confirm which behavior pydantic already provides, remove the redundant __repr__ and __eq__ methods, and verify that the existing simulation-input behavior remains unchanged.

Written by the indexing model from the issue text.