Update and fix fred-oil-brent-wti.ipynb per issue #2.
New shortcut: https://git.io/oil for crude oil markets.
New introductions regarding petrochemicals
and the Boltzmann portfolio of oils.

nb/fred-debt-pop.ipynb: Update preamble and data.
Use gemrat() instead of georet().
Append Appendix 1. New shortcut: https://git.io/debtpop

fecon235.py: Add foreholt() function which
supercedes yi_fred.holtfred(), but
move holtfred() to fecon235 module for backward compatibility.
Then at lib/yi_fred.py: comment out holtfred() with notification.
We only need to maintain foreholt().
Pre-2016 notebooks with updated import and preambles should work.

Fix fred-inflation.ipynb per issue #2, and optimize forecast.
Holt-Winters model now uses robust optimized parameters.
Unified inflation has its own section.
Geometric mean rate introduced.
Drop section on correlation to gold.
Summary combines three orthogonal forecasts.

fecon235.py: Add foreinfl() to forecast Unified Inflation.
The best documentation for this function is nb/fred-inflation.ipynb
which shows how it was derived by interacting with data and plots.
This single function distills the forecasting process
derived in the notebook. Further discussed in new Appendix 2.