Data science up for first round of review

1) Focus on rspec tests 
2) Stuck at trying to refactor the exception part
ambiguous calculation methods. If using a cohort class @coversion_rate, @standard_error, @confidence can be methods properties of a 'Cohort' parent class. Also since you have methods with the same names as the class attributes, you can do away with using @variables altogether and just call the class methods directly