Add more boundary models

**Is your feature request related to a problem? Please describe.**
Newer boundary models exist and can be added to OCBpy.

**Describe the solution you'd like**
- [ ] Troyer model
- [x] Gussenhoven model

**Describe alternatives you've considered**
Will also add my model, once it's created.

**Additional context**
Adding existing models will be useful for my research workflow.