Adding notebook and codes for Krusell and Smith (1998)

[albep]

This PR adds the directory krusell_smith_98_deps in dependencies. Such directory contains 2 Julia files with the programs to solve the model and replicate some results from Krusell and Smith (1998). These programs are run in the notebook krusell_smith_98.ipynb - also added through this commit - with extensive descriptions.

[jstac]

@albep Great. This is very nice.

@mmcky How would you like to handle these dependencies?

[mmcky]

Given this is a Julia project -- and the current notebook dependency tools support python only -- for now we should just add them in the dependencies folder and then ask Julia to include them from the github repo url directly. I will check with Spencer for the appropriate snippet. We will need to add a utility to QuantEcon.jl

[sglyon]

I think this is a great time to write up some code for a proper solution in Julia. @mmcky can you summarize all the things the python dependency system currently does?

[mmcky]

It is pretty basic -- it just uses requests to fetch a file and place it in the cwd.
It is written on the keep it simple philosophy for now. It is not automatic and requires a single line specifying the file name -- and that file needs to be in QuantEcon.notebooks/dependencies

import os
import requests

#-Remote Structure-#
REPO = "https://github.com/QuantEcon/QuantEcon.notebooks"
RAW = "raw"
BRANCH = "master"
DEPS = "dependencies"          #Hard Coded Dependencies Folder on QuantEcon.notebooks


def fetch_nb_dependencies(files, repo=REPO, raw=RAW, branch=BRANCH, deps=DEPS, overwrite=False, verbose=True):
    """
    Retrieve raw files from QuantEcon.notebooks or any other Github repo

    Parameters
    ----------
    file_list   list or dict
                A list of files to specify a collection of filenames
                A dict of dir : list(files) to specify a directory
    repo        str, optional(default=REPO)
    branch      str, optional(default=BRANCH)
    deps        str, optional(default=DEPS)
    overwrite   bool, optional(default=False)
    verbose     bool, optional(default=True)

    TODO
    ----
    1. Should we update this to allow people to specify their own folders on a different GitHub repo?

    """

    #-Generate Common Data Structure-#
    if type(files) == list:
        files = {"" : files}

    #-Obtain each requested file-#
    for directory in files.keys():
        if directory != "":
            if verbose: print("Parsing directory: %s")
        for fl in files[directory]:
            if directory != "":
                fl = directory+"/"+fl
            #-Check for Local Copy of File (Default Behaviour is to Skip)-#
            if not overwrite:
                if os.path.isfile(fl):
                    if verbose: print("A file named %s already exists in the specified directory ... skipping download."%fl)
                    continue
            else:
                if verbose: print("Overwriting file %s ..."%fl)
            if verbose: print("Fetching file: %s"%fl)
            #-Get file in OS agnostic way using requests-#
            url = "/".join([repo,raw,branch,deps,fl])
            r = requests.get(url)
            with open(fl, "wb") as fl:
                fl.write(r.content)