Improve notebook dependencies fetch utility

[mmcky]

1. Incorporate a cache mechanism so that files don't always need to be fetched (using contents of files)

[mmcky]

Currently the implemented nb_fetch_dependencies() function is a simple utility that defaults to fetching files from dependencies folder in QuantEcon.notebooks repo. It does also allow fetching files from other locations such as QuantEcon.applications but becomes more difficult to specify with repo= and folder arguments etc. when the files live in other locations. This works for now and @oyamad has used this utility in a recent notebook - but could be improved upon.

We will stick with this for the time being - but we could improve this utility:

1. swap to using https://github.com/ellisonbg/antipackage (with Cache features etc) [Con: files then get stored in a user cache and isn't directly accessible to the end user, Pro: has cache support etc]
2. improve the current nb_fetch_dependencies() method
3. explore constructing a Jupyter magic command that could import the dependency file(s) from a web location.

[mmcky]

The current utility has been generalized and this issue can be closed.