write BoxSize as vector.

Former-commit-id: 449e4644b3b3aa68611cb7ac6a1da1dc69aaca7a

src/serial_io.c

@@ -445,7 +445,7 @@ void write_output (struct engine *e, struct UnitSystem* us)
     error("Error while creating file header\n");
     
   /* Print the relevant information and print status */
-  writeAttribute(h_grp, "BoxSize", DOUBLE, e->s->dim, 1);
+  writeAttribute(h_grp, "BoxSize", DOUBLE, e->s->dim, 3);
   writeAttribute(h_grp, "NumPart_ThisFile", UINT, numParticles, 6);
   writeAttribute(h_grp, "Time", DOUBLE, &e->time, 1);