Update broken links and add cython to environment.yml

[hodgestar]

Minor fixes:

• Update the nbviewer links to use nbviewer.jupyter.org (nbviewer.ipython.org is deprecated and no longer works).
• Update the dead quantum teleportation reference with the Wikipedia article.
• Add Cython to the environment.yml, which is slightly friendlier to developers who might expect Cython to be installed when editing tutorials (see Add cython to environment.yml. #46).

[hodgestar]

@christian512 There are three strange failures in this PR that are unrelated to the PR itself. Any chance you could take a look?

[christian512]

After a re-run of the jobs, it looks like there is only a problem with pdflatex. The CI installs the package texlive-latex-extra. This also includes the installation of texlive-picture, which contains the required qcircuit package.

I am not sure if there were recent changes in qutip-qip that broke this plotting function. @BoxiLi, do you have an idea?

[BoxiLi]

Okay, I'll have a look. Let me try testing pdflatex directly in the command line on the GitHub action machine.

[christian512]

I ran pdflatex manually in a GitHub action in this job: https://github.com/christian512/qutip-tutorials/actions/runs/3426603357/jobs/5708606925

There is an error with Xy-pic, which I don't understand. Maybe you can take a look.

[BoxiLi]

Okay, I tried it locally. The generated tex code is wrong (should have checked it earlier...). This seems to be related to qutip/qutip-qip#166.

[BoxiLi]

I pushed a fix on qutip/qutip-qip#177

[BoxiLi]

Seems the circuit plotting error is gone now but there are a few others.

[hodgestar]

+1 -- thanks @christian512 for fixing all the little bugs that crept in. We should add the nightly build so that these don't sneak in so easily in future.