Adds divisor methods.
Moves divisors method to new NumTheory class, also adds abundance method.
Adds versions of abundance() to give pre-calculated divisors if they've already been found by the divisors() method or some other way.
Adds primeFactors() and gcd().
Adds tests for NumTheory and test for MathUtils factorial().
Adds factorial().
Adds case to gcd() for when n or m is 0.
Adds more checks for non-positive input on functions.
Adds Fraction class.
Adds Fraction test.