Session02

Jesse Klein's Ackermann's function module
Series module added as well
I believe that using an `if...elif...else` set of conditionals instead of three separate `if` statements results in one less comparison, because the code in the `else` block runs as as soon as the expressions after `if` and `elif` evaluate to false. Hopefully, @cewing can confirm  this or correct me. Either way, it's a minor issue for this assignment, but for other uses it'll be important to know the difference between subsequent `if` statements and a set of `if...elif...else` statements.