Make small changes

include/PenningTrap.hpp

@@ -13,9 +13,13 @@
 #define __PENNING_TRAP__
 
 #include <armadillo>
+#include <omp.h>
 
-#include "constants.hpp"
 #include "Particle.hpp"
+#include "constants.hpp"
+
+#pragma omp declare reduction( + : arma::vec : omp_out += omp_in ) \
+  initializer( omp_priv = omp_orig )
 
 /** @brief A class that simulates a Penning trap.
  *