Added notebook in examples explaining the functionality of wigner_tra…

[NithinRamu]

…nsformation and plot_Wigner_sphere in wigner.py and visualization.py respectively in my fork of qutip, for which I have just created a pull request.

[nathanshammah]

Minor enhancement proposals:

• replacing \uparrow with \\uparrow (and so on) might fix the LaTeX rendering (might be just a Github issue)
• consider importing all libraries at the beginning of the first block
• "represenation" -> "representation"
• could add a comment on the result of the Wigner sphere plot
• add and run qutip.about() at the end of the notebook
  Thank you for this notebook.

[nathanshammah]

• run the qutip.about() block, such that it will print the specifics of the relative QuTiP installation.
• to render bold text, change \textbf{Special thank you to Russell P Rundle} to **Special thanks to Russell P Rundle** (no space between the text and the asterisks)
• apologies, placing two backslash $\\uparrow$ etc. did not work. my bad. Some Latex text in notebook might not render properly on Github, but then is fine on the website. Please change back the Bell state to $\uparrow[...]$.
• if you can, comment on the meaning of the plotted Bloch sphere under the plot.
• you can render code font, such as fullparity, slicearray by typying it it like this `slicearray`
• [] $\left|\rm up \right>$ and $\left|\rm down \right>$ might refer to $\left|\rm \uparrow \right>$ and $\left|\rm \downarrow \right>$?

[nathanshammah]

There is an error message in the notebook. I would be glad to merge this if these errors can be fixed (best if we can address also the points above). This way we can include it also in the next QuTiP version release coming soon.