Logger dimensionality compilation error fix

Made sure logger_particle vector variables have size 3 to fix a compilation error when configuring with hydro dimension =/= 3.

Since we do not expect to ever use the logger for very large 1D or 2D simulations, I just changed the array sizes to 3, irrespective of the actual number of dimensions. This causes a minor memory overhead. The alternative solution would have been to explicitly check the dimensionality whenever accessing these arrays and that would have been ugly.

logger/logger_particle.h

@@ -53,13 +53,13 @@ struct logger_reader;
  */
 struct logger_particle {
   /* position. */
-  double pos[DIM];
+  double pos[3];
 
   /* velocity. */
-  float vel[DIM];
+  float vel[3];
 
   /* acceleration. */
-  float acc[DIM];
+  float acc[3];
 
   /* entropy. */
   float entropy;