Fixed KeyError, addressed speed problems

Fixed an error where stochastic_action_selection was not properly ignori...ng None values (leading to a negative scope and KeyError)

Addressed speed problems:
Switched masks over to index masks instead of boolean masks
Push, pop masks by h-stacking and sub-listing instead of applying masks to an index array
Removed the 'abundance' property from FeatureRecords. Instead, abundances should be accessed directly from ProblemData when needed.