Exercise: Step 3 break out the context map initialization

[pberkes]

• Move the context map initialization to 3 functions in a separate context_maps.py module
• Modify the constructor of Walker to take a context_map array instead of a map_type
• Modify the notebook to use the new code

Remember to reload the notebook kernel when you modify the code in you modules, as modules are cached during the first import and changes would not be visible.