Renommer les fichiers "planet" en "celestial_body"

practical_work/planets/skeleton/planet/planet.c → practical_work/planets/skeleton/planet/celestial_body.c

-#include "planet.h"
+#include "celestial_body.h"
 #include <stdlib.h>
 
 #define G 6.67e-11

practical_work/planets/skeleton/planet/planet.h → practical_work/planets/skeleton/planet/celestial_body.h

-#ifndef _PLANET_H_
-#define _PLANET_H_
+#ifndef _CELESTIAL_BODY_H_
+#define _CELESTIAL_BODY_H_
 
 #include "../vec2/vec2.h"
 #include "../gfx/gfx.h"