Create a tenant class

* Has name, age, and credit score
Create credit rating instance method
Create an apartment class

* Has a number, rent, square footage, number of bedrooms, and number of bathrooms
Create an instance variable for tenants that cannot be modified directly
Create add tenant method
Raise errors for add_tenant method

* Error if potential tenant has a bad credit rating
* Error if apartment is full
Create remove tenant class

* Takes name or object as parameter
* Raises error if tenant not found
Create instance method to remove all tenants
Create apartment credit score method
Refactor code to add credit_rating method to a module
Refactor apartment class to remove duplicate getter methods
Create building class that has an address
Building has an array of apartments
Create method to add apartments
Create a method that removes an apartment by number
Create a sq_ft method that calcs sq ft of building
Create a revenue method for building class
Create instance method in building to list all tenants
Create method to retrieve all apartments grouped by credit rating