Documentation issues

[volkm]

Keeping track of documentation issues and points of improvements.

See #244 for pybind11-specific issues.

List of related issues:

• Add type information for Python wrapper #240

• Ensure consistency between Jupyter notebooks and scripts in examples folder #256

• Hidden classes have no visible API documentation. As a result subclasses then also have no documentation on the inherited methods. For instance, there is no documentation on initial_states because it is part of _SparseModel.