use c++ style system headers

src/MANYBODY/pair_atm.cpp

@@ -15,10 +15,10 @@
    Contributing author: Sergey Lishchuk
 ------------------------------------------------------------------------- */
 
-#include "pair_atm.h"
 
-#include <math.h>
+#include <cmath>
 
+#include "pair_atm.h"
 #include "atom.h"
 #include "citeme.h"
 #include "comm.h"