d_evans_parallel_computing: added .py file appendix, fixed typos.

Hi everyone and happy new year!

Great project. Thank you for sharing it. This project was a key driver for me in deciding to make the switch from matlab/fortran to python.

I plan to do some work with heterogeneous agents so I read the parallel computing notebook with great interest. It had a bunch of spelling errors, and I hope I got them all. Moreover, to be useful to someone just browsing the site, and for the sake of full documentation, the notebook has to be able to show and generate all the .py files discussed. I put them all on an appendix with `%%writefile` magics.

There may be some noise in the diff due to the fact that I work with the ipython dev version and had to port the notebook back from v4 format to v3. Someone with ipython stable should make sure nothing broke.