Data-Science Code Review

Joe,
I forked level_up_exercises and imported solutions that I had been placing in a different project. Going forward I will use this fork to submit pull-requests.

Zamir
these are really tests on `Interval` ... you might want to move the `lower`/`upper`/`midpt` expectations into a separate spec on `Interval`, and within this spec just test that `Confidence#interval` returns an `Interval` object