Allow to implement Python bindings with pybind11 (start with mixture module)

[amgebauer]

This PR adds to possibility to add python bindings to 4C (similar to unit- or benchmark tests).

Draft for not since we need to figure out how we want to structure the directories. Where should we put the bindings (and the respective tests?)

Maybe something like this (once we have #81):

4C/
├── core/
│   ├── discretization/
│   │   ├── benchmark_tests/
│   │   ├── python/
│   │   |   ├── bindings/
│   │   │   └── tests/
│   │   ├── src
│   │   ├── tests
│   │   └── ...

For our legacy modules, we need a top-level directory for all python bindings (I currently call it pybind11)

[Copilot]

Pull Request Overview

This PR introduces Python bindings infrastructure for the 4C multiphysics library using pybind11, starting with the mixture module. The implementation creates a foundation for exposing C++ classes and functionality to Python through automatically generated bindings.

Key changes include:

• Implementation of a submodule registry system for organizing Python bindings
• Python bindings for mixture module fiber materials and remodel implementations
• CMake infrastructure for automatically generating pybind11 modules

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
pybind11/setup.py.in	Template for Python package setup configuration
pybind11/mixture/CMakeLists.txt	CMake configuration for mixture module bindings
pybind11/mixture/4C_mixture.cpp	Python bindings implementation for mixture fiber classes
pybind11/config/CMakeLists.txt	CMake configuration for pybind11 config library
pybind11/config/4C_config_submodule_registry.hpp	Header defining submodule registry system
pybind11/__init__.py.in	Template for Python package initialization
pybind11/CMakeLists.txt	Main CMake configuration for Python bindings
pybind11/4C_main.cpp	Main module definition and submodule initialization
cmake/functions/four_c_auto_define_pybind11_module.cmake	CMake function for automatic module definition
CMakeLists.txt	Root CMake file with pybind11 subdirectory inclusion

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[sebproell]

Looks interesting!