Add percentage of atoms within cutoff radius when using LAMMPS reference version

Signed-off-by: Rafael Ravedutti <rafaelravedutti@gmail.com>

lammps/includes/stats.h

@@ -28,6 +28,8 @@
 typedef struct {
     long long int total_force_neighs;
     long long int total_force_iters;
+    long long int atoms_within_cutoff;
+    long long int atoms_outside_cutoff;
 } Stats;
 
 void initStats(Stats *s);