To update pre-2016 notebooks, fix broken import and preamble

[rsvp]

Major release v4.15.1230 benefits from the python3 compatibility changes
made during v3. All modules are now operational under
both Python 2 and 3. Also, code has been rewritten for
cross-platform performance (Linux, Mac, and Windows).

We MOVED the yi-modules from nb to a new directory: lib.
Python 3 uses absolute import and our python2 code
now conforms to that practice.

To update pre-2016 notebooks, please use import style
discussed in docs README: https://git.io/fecon-intro
The top-level module fecon235.py
(formerly known as nb/fecon.py) is also
explained in that introduction.
With adoption of python3 print_function,
the python2 print statement must be rewritten as a function.

We also highly recommend inclusion of PREAMBLE-p6.15.1223
which gives versioning requirements for successful
notebook replication. With those fixes, our notebooks
should run under both Python kernels in Jupyter,
without any dependency on Linux.

[rsvp]

Here's another illustration of "Fix issue #2 with v4 and p6 upgrades":
https://git.io/georet which concerns comparative geometric mean returns.

Notice how the import cell precedes the preamble cell in the notebook.