Specify pathpyG design principles

[IngoScholtes]

Specify design principles in readme.md

[M-Lampert]

I've been meaning to write a Jupyter notebook (see here) that explains the concepts that are now used for the order lifting and so on. Maybe we can combine both in one Tutorial notebook that explains all the technical concepts and design principals that we use. I think @VincenzoPerri is currently working on something like this.