define tenant and apartment classes
implement add_tenant with credit and available room checks
implement remove tenants with error if tenant is not present
implement avg_cred for an apartment
implement credit rating for apartments
define building class
implement add and remove apartments for building class
implement sq_footage calculation for building class
add tenants reader to apartment, use to implement tenants function for building
implement group by credit rating, returns array rate => [apartments]
add rent field to apartment and apartment initializer
implement monthly_revenue method
include empty apartment and eviction check/errors to remove_apartment