Add a function that takes a named feature and plots the area around it

[Christian J. Tai Udovicic]

This issue builds off of #2 and #3. You may want to wait until those are finished to take this one on.

Write a function in helpers.py that:

• Takes the name of a named feature from named_features.csv as an argument
• Gets the center lat, center lon, and diameter of that feature and converts these into a lat/lon extent
• Uses the min / max lat and lon to plot the area around that feature

Add an example in jupyter_notebooks/ (you can edit tutorial.ipynb or make your own notebook) which shows your function in action, plotting multiple named areas on Pluto

[heshanipieris]

I'll work on this!