Data science up for first round of review

1) Focus on rspec tests 
2) Stuck at trying to refactor the exception part
Since this class acts solely on specific Cohort data, their functionality *can* (doesn't have to) be encapsulated into a cohort class itself. If the idea is to separate calculation logic from base class, this can be done either through inheritance `class Cohort < CohortCalculator` or turn it into a module that can be extended/included inside of a Cohort class. Also class name should be StatisticsCalculator for readability