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

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