scrap "alternative approaches" in extras section, add "intro to pandas and data frames"

"Alternative approaches" just contains a couple short blurbs about SQL and Pandas - far too little. Pandas DataFrames are essential for DS, so there should be at least one section dedicated to this topic. I would suggest breaking up it up further, e.g.:

• Intro to Pandas DataFrames (What is Pandas? What are data frames? Some basic functions and methods that clarify the use case for DFs.)
• Importing data with Pandas
• Basic stats with Pandas (already an issue)
• …

An intro to basic SQL could be added to extras later.

[ellenkoenig]

Idea: Start off by re-doing the "radish analysis" in pandas as an intro covering exactly what you outlined: http://opentechschool.github.io/python-data-intro/core/survey.html