The harmonic-add function takes in a start and an end number,
and finds the sum 1/start + 1/(start+1) + ... + 1/end.
(check-expect (harmonic-add 1 2) 1.5)
(check-within (harmonic-add 1 3) 1.8333 0.0001)
(check-within (harmonic-add 1 4) 2.0833 0.0001)