Restore the union in the DEFAULT_SPH particle definition

src/const.h

@@ -56,8 +56,8 @@
 
 /* SPH variant to use */
 //#define MINIMAL_SPH
-//#define GADGET2_SPH
-#define DEFAULT_SPH
+#define GADGET2_SPH
+//#define DEFAULT_SPH
 
 /* Gravity properties */
 #define EXTERNAL_POTENTIAL_POINTMASS

src/hydro/Default/hydro_part.h

@@ -69,7 +69,7 @@ struct part {
   float alpha;
 
   /* Store density/force specific stuff. */
-  // union {
+  union {
 
     struct {
 
@@ -105,7 +105,7 @@ struct part {
       float c;
 
     } force;
-  //};
+  };
 
   /* Particle mass. */
   float mass;