Rename tuple file to remove the plural.

Also add empty shell ray.

source/tuple.cpp

@@ -0,0 +1,35 @@
+/*
+ *  DoRayMe - a quick and dirty Raytracer
+ *  Tuples implementation
+ *
+ *  Created by Manoël Trapier
+ *  Copyright (c) 2020 986-Studio.
+ *
+ */
+#include <tuple.h>
+
+#include <math.h>
+
+
+double Tuple::magnitude()
+{
+    return sqrt(pow(this->x, 2) + pow(this->y, 2) + pow(this->z, 2) + pow(this->w, 2));
+}
+
+Tuple Tuple::normalise()
+{
+    double mag = this->magnitude();
+    return Tuple(this->x / mag, this->y / mag, this->z / mag, this->w / mag);
+}
+
+double Tuple::dot(const Tuple &b)
+{
+    return this->x * b.x + this->y * b.y + this->z * b.z + this->w * b.w;
+}
+
+Vector Vector::cross(const Vector &b) const
+{
+    return Vector(this->y * b.z - this->z * b.y,
+                  this->z * b.x - this->x * b.z,
+                  this->x * b.y - this->y * b.x);
+}