gcd(a,b) function that the first argument (a) must be less than the second
argument (b). To be sure our fraction reducer will always work, let's write a "wrapper function" called get_gcd that will
ensure that the main gcd function gets called with the lower number first.
if statement to compare a and b, and the two gcd function calls to ensure that
gcd(a,b) always gets called with $a< b$.