most-minor-cells should ignore white cells
rename most-minor-cells to most-minor-agents
refactor most-minor-agents using for/list
add find-free-spaces-incrementally
add random-member
add movable?
most-minor-agents: agents must be movable
add decide-cells-to-swap*
fix bug in decide-cells-to-swap* when min is applied to '()
add change-world-once!
add change-world!
make gui work
create a hard-coded version of create-world