implements class variables in place of constants, and rotates through the questions array to limit duplicate questions
began data structure assignment prompt
creates a subclass for VariableQuestion
adds initial tests of subclass DataStructureAccessQuestion
created subclasses for DataStructureQuestion and NestedDataStructureQuestion
adds comment on use of class instance variable in NestedDataStructureAccessQuestion
gets data structure assignment working
removes unecessary conversion of class to string