Added an expository TODO.

NumberTheory/Factors.hpp

@@ -16,6 +16,7 @@ namespace NumberTheory {
 		@returns The greatest common divisor of @c a and @c b as computed by Euclid's algorithm.
 	*/
 	template<class T> T greatest_common_divisor(T a, T b) {
+		// TODO: replace with the C++17 GCD function, once available.
 		if(a < b) {
 			std::swap(a, b);
 		}