Review indexing material

[dcherian]

From review of #192

• let's delete the numpy advanced indexing piece from "basic indexing" material. That's going to be confusing. It can be moved to the "advanced indexing" notebook if you prefer.
• "basic indexing" uses datetime indexing before it is introduced. Let's edit that to select along lat instead of time. or reorder the material

[scottyhq]

closed by #201