Add stopping power for protons in liquid water according to one of the models

[grzanka]

Needs #107 first

Proposition for package structure:

stopping/
├── __init__.py
├── power/
│   ├── __init__.py
│   ├── electronic.py
│   ├── nuclear.py
│   └── total.py
├── ranges/
│   ├── __init__.py
│   ├── electron.py             # ← moved electron range logic here
│   └── csda.py
├── beam_models/
│   ├── __init__.py
│   ├── bortfeld_proton.py
│   └── wilkens_proton.py