fix C2H5OH formula
#3 add TODO, noting that Reaction_P4_10Cl2__4PCl5 causes problems with challenge generation
rename RPALConstants.COEFFICIENT_RANGE to SANDWICH_COEFFICIENT_RANGE
bug fix
lint fixes
remove debugger
 replace ChallengeVisibility enum with 2 booleans
make GameModel extend PropertySet
first pass at level-selection interface for Game