Add Write methods to scene objects

Write methods are polymorphic, overridden by subclasses. There is a single operator<< for Objects that calls the right Write() method for the object. Neat.

src/object_sphere.h

@@ -29,6 +29,10 @@ public:
     bool DoesIntersect(const Ray& ray, TVector& t, Stats& stats) const;
     bool point_is_on_surface(const Vector3 &p) const;
     Vector3 compute_normal(const Vector3 &p) const;
+
+    /** @see charles::Object::Write */
+    void Write(std::ostream& ost) const;
+
 private:
     float radius;
 };