Update README and add norm calculations to vector

include/vec3.h

@@ -1,6 +1,7 @@
 #ifndef VEC3_H
 #define VEC3_H
 
+#include <cmath>
 template <typename T> struct Vec3 {
   T x;
   T y;
@@ -20,14 +21,18 @@ template <typename T> struct Vec3 {
     z *= scalar;
   };
 
-  inline T dot(Vec3<T> other) {
+  inline T dot(Vec3<T> other) const {
     return x * other.x + y * other.y + z * other.z;
   }
 
-  inline Vec3<T> cross(Vec3<T> other) {
+  inline Vec3<T> cross(Vec3<T> other) const {
     return {y * other.z - z * other.y, z * other.x - x * other.z,
             x * other.y - y * other.x};
   }
+
+  inline double squared_norm2() const { return x * x + y * y + z * z; }
+
+  inline double norm2() const { return std::sqrt(squared_norm2()); }
 };
 
 #endif