simplify double map into single map

src/physics.rs

@@ -877,8 +877,7 @@ impl Body{
 			T1:integer::Fix<Planar64>,
 	{
 		// a*dt + v
-		self.acceleration.map(|elem|dt*elem)
-		.map(|elem|elem.divide().fix())+self.velocity
+		self.acceleration.map(|elem|(dt*elem).divide().fix())+self.velocity
 	}
 	pub fn advance_time_ratio_dt(&mut self,dt:model_physics::GigaTime){
 		self.position=self.extrapolated_position_ratio_dt(dt);