Basics notebooks

[christian512]

Adds notebooks from the Basics Section on the QuTiP tutorial website.

[hodgestar]

I wonder if we shouldn't scrap the basics section entirely and move the two notebooks elsewhere. The superoperator tensor contracting is definitely not "basics", and neither is the ultrastrong coupling groundstate.

We can perhaps replace it with a paragraph on the index page which suggests some places to start.

[christian512]

• I wanted to move the images out of the tutorials-vX directories, so I don't have to add the images twice in the repository. I understand that having them close to the notebook helps with readability, but having the same images twice in the repository is also unnecessary.
• the ultrastrong-coupling-groundstate was just moved from tutorials-vX to tutorials-vX/basics.

[hodgestar]

I think we can live with having two copies of the images in order to have the tutorial-v4 and tutorial-v5 folders be a bit more independent.

As an example, in try-qutip, I copy only tutorial-v4 and name it tutorial since only qutip 4 is installed, but with the images moved out this will now break. I could copy images too, but the point is that having the notebooks depend on files outside the directory subtree the notebook appears in is a bit messy for everyone trying to use a subset of the notebooks or even a single notebook.

Image that you want to download and use just one notebook. Previously you could download or copy the subfolder or just the notebook and the images and it worked. Now you have to download the whole repository structure, and you have to edit the notebook if you want to move it anywhere else.

[christian512]

@hodgestar, thanks for taking a look! The file is unused in the lectures.

I'm working on a fix for the recent failure of the tests, due to some LaTeX problems. I will merge into this branch once it's done.