[WIP] restructure of the model Framework

This is my idea of a restructure of the model framework. Currently it showcases an easy way to attach different attributes to models and link them (`density0` will be scaled to density when accessing `.density`). This will make it easy to store the models in dataformats like HDF5, make them more modular. This also includes some documentation (to see how they work) and tests

**TODO**
- [x] preliminary overview with some documentation and testing
- [ ] implement docstrings for the attributes
- [ ] implement HDF5 reader writer