Updated pandas shape notebook

[sglyon]

• Added example of reset_index where levels are index names, not just integers. It felt like it should be something we show instead of an exercise
• Reworded a few things: cut out some text that was either confusing or not necessary
• corrected some typos
• Added comment about looking at the levels and their names in a multi-index
• Added section on the xs method
• Removed some 1/2 finished thoughts