densitiesCheckBox is now a LinkedHashMap. Closes #6

You have a "random" density list because HashMap is a data structure that doesn't maintains insertion order (this allows a O(1) get/put computational time). I simply changed it to a LinkedHashMap which maintains insertion order (via LinkedList). While this fixes the issue, we can always provide a better implementation to avoid *HashMap data structures. But this is just trivial and I think other issues/features have higher priority.