Updated the intro notebook to reflect current repo
Updated the intro notebook to reflect current repo

A cleaner, more complete, and hopefully clearer `pudl_intro.ipynb`
notebook, that ought to be correct as of today.

Includes:
 * importing `pudl` directly.
 * pulling a raw FERC database table (`f1_fuel`).
 * pulling the PUDL version of the same table (`fuel_ferc1`).
 * pulling whole data tables, and also using SQL queries through
   `pd.read_sql()`.
 * using PudlTabl tabular output objects to pull human readable tables.
 * using PudlTabl tabular output objects to calculate marginal fuel
   costs.
 * making some plots based on MCOE calculations.

Closes #298

catalyst-cooperative/pudl,catalyst-cooperative/pudl