Re-instated long-range truncation for periodic gravity.

src/kernel_long_gravity.h

@@ -37,15 +37,14 @@
 __attribute__((always_inline)) INLINE static void kernel_long_grav_eval(
     float u, float *const W) {
 
-  /* const float arg1 = u * 0.5f; */
-  /* const float arg2 = u * one_over_sqrt_pi; */
-  /* const float arg3 = -arg1 * arg1; */
+  const float arg1 = u * 0.5f;
+  const float arg2 = u * one_over_sqrt_pi;
+  const float arg3 = -arg1 * arg1;
 
-  /* const float term1 = erfcf(arg1); */
-  /* const float term2 = arg2 * expf(arg3); */
+  const float term1 = erfcf(arg1);
+  const float term2 = arg2 * expf(arg3);
 
-  /* *W = term1 + term2; */
-  *W = 1.f;
+  *W = term1 + term2;
 }
 
 #endif  // SWIFT_KERNEL_LONG_GRAVITY_H