Data science/mabdeljalil V1

Eventually, I think this would become non-performant. Really, I'm not sure there's anything useful that a class at this granularity is doing. You could refactor it into `Visitor = Struct.new(:cohort, :visit_date, :result)`.

Or, alternatively, if we ever added functionality to filter visits, you'd end up needing it after all. So there's that. Just something to be aware of.