Add mother class and belongs_to association between a person and their mother.
Make mother class inherit from person so that a mother can have a spouse.
WIP implementing mother.
Remove mother class. Make person belongs_to a mother, also of class person.
Remove mothers table.
Finish implementing add_mother method in ui.
Add error message to list_people method for if there are no people.
Add Location class with locations table and many-to-many join table for locations and people.
Add test for has_and_belongs_to_many people association for locations.
WIP add ui methods for locations.