Remove 8-cell limit in gravity + non periodic case

Fix issue #323 (closed)

src/space.c

@@ -314,14 +314,22 @@ void space_regrid(struct space *s, int verbose) {
         "small,\n"
         " - the (minimal) time-step is too large leading to particles with "
         "predicted smoothing lengths too large for the box size,\n"
-        " - particle with velocities so large that they move by more than two "
+        " - particles with velocities so large that they move by more than two "
         "box sizes per time-step.\n");
 
-  /* Check if we have enough cells for gravity. */
-  if (s->gravity && (cdim[0] < 8 || cdim[1] < 8 || cdim[2] < 8))
+  /* Check if we have enough cells for periodic gravity. */
+  if (s->gravity && s->periodic && (cdim[0] < 8 || cdim[1] < 8 || cdim[2] < 8))
     error(
-        "Must have at least 8 cells in each spatial dimension when gravity "
-        "is switched on.");
+        "Must have at least 8 cells in each spatial dimension when periodic "
+        "gravity is switched on.\nThis error is often caused by any of the "
+        "followings:\n"
+        " - too few particles to generate a sensible grid,\n"
+        " - the initial value of 'Scheduler:max_top_level_cells' is too "
+        "small,\n"
+        " - the (minimal) time-step is too large leading to particles with "
+        "predicted smoothing lengths too large for the box size,\n"
+        " - particles with velocities so large that they move by more than two "
+        "box sizes per time-step.\n");
 
 /* In MPI-Land, changing the top-level cell size requires that the
  * global partition is recomputed and the particles redistributed.