refactor: Rename src directory to root_painter_trainer

[jakob1379]

This PR renames the python module src to root_painter_trainer. This allows build systems to pick up the package automatically.

Key Changes:

• Package Layout: The source code has been moved from src directory to a top-level root_painter_trainer package directory. This is a common simplification for Python packaging.
• Configuration Update: trainer/pyproject.toml has been updated to reflect the new package structure by removing the package-dir setting. The readme has also been corrected to README.txt.

This is also in compliance with PEP8 (style guide) and PEP420 (Implicit Namespace Packages)

[Abe404]

Thanks for the PR. I see the motivation for renaming src/, and it might make sense as part of a broader refactor or packaging improvement. But right now, I'm not planning any major structural changes, and I don’t want to introduce changes unless I fully understand the benefits to users and maintainability myself.

Renaming this folder may require updating docs, the Colab notebooks (linked in the main README), and probably the packaging setup? I’m not personally confident all the knock-on effects are accounted for yet.

Perhaps we should chat at some point about the overall goals with your changes, and maybe we could work together on a more concrete proposal or roadmap for where this is heading, but for now I’d prefer to hold off.