Replace model by `pathsim`

[RemDelaporteMathurin]

We should make use of @milanofthe 's great package pathsim https://github.com/milanofthe/pathsim and replace what we currently have in https://github.com/LIBRA-project/libra-toolbox/blob/main/libra_toolbox/tritium/model.py

[RemDelaporteMathurin]

This also links to #16 by allowing several species to flow through the system. Consider this example