Port notebooks from qutip-notebooks

[christian512]

To add content to this repository, we have to port the notebooks from the qutip-notebooks repository to this one. During the port, we should modify the notebooks such that they work with QuTiP 4 and 5, and add some documentation (markdown).

To keep track of the progress, we use the following GitHub project: https://github.com/users/christian512/projects/1/