WIP: Add Julia/Python code with 7 solutions to growth model

[cc7768]

Related to #53

These notebooks describe and compare various solution methods in the framework of a Neoclassical growth model. Currently only the Julia version is written out, but it is just a matter of changing some words and including the (already written) code to get the Python version ready.

Notebooks also in part serve as an appendix for a working paper with @sglyon and @maliars.

[jstac]

@cc7768 Thanks.
@cc7768 @sglyon A tangential comment: From now on I suggest that we switch to a model where we simply post notebooks when we're ready to do so, without anticipating feedback or corrections, since that's how the new site is going to function. I'll simply merge any PR to this repo as soon as it comes in.

Edit: Having said that, I meant to ask, is this PR good to go?

[cc7768]

@jstac Good to know -- I'll only post once ready from now on.

It might make sense to wait to merge this until I have the Python notebook ready. That should be in the next day or two.

[jstac]

@cc7768 What would you like me to do with this PR? (I guess this repo will be closed soon...)