Update ddp_ex_career_py for QE v.0.3

[oyamad]

Add script that downloads career.py from applications repo

See cell [45], where I basically mimiced fetch_nb_dependencies. I hope the code there is properly written.

[oyamad]

I realized fetch_nb_dependencies is flexible enough to accommodate any repo. See cell [44] in the updated version.

[mmcky]

Thanks @oyamad. Given the relocation of the models code this has rendered applications like this a bit more difficult. Perhaps we should revisit setting up a quantecon.models package to make the demo models importable through a package? I'll add this to the list of discussions tomorrow.

[mmcky]

We can discuss this possibility at tomorrow's meeting. The notebook works as is so I will close this and merge the updates. An alternative to a models subpackage is that we could use: antipackage to construct a module cache (as noted by @oyamad).