Create test.rb and building.rb.
  *initialize methods for building.rb
Add apartment class nad tenant class.
Tenant class - initialized and add credit_check method.
Add add_tenant method for Apartment class.
Add delete_tenant method in apartment
Modify delete_tenant method
Add delete_all_tenant
Add credit_average and credit_check methods
Add add_apartment , delete_apartment methods
Small fix in apartment.rb. Add add_apartment & delete_apartment methods.
Modify delete_apartment method
Add total_square_footage method
Add total_monthly_revenue method
Add Tenant_list. Modify add_apartment
Add group_by_credit method. Modify test.rb.