Reformat return arrays.
Add functions to get mortgage and house cost.
Fix syntax error in setUserCash.
Add function to add user cash.
Fix cash add method to return either the new cash or false.
Implement mortgaging property.
Better check constraint for the number of houses.
Add function to set the number of houses on a property.
Add function to get all space ids in group.
Add rule for enforcing parallel improvements.
Implement selling houses.
Set user cash upon selling houses.
Add function to get cost of improving group.
Implement buying houses.