minor change for consistency

lib/nBodySim.hpp

-#ifndef NBODYSIM_H
-#define NBODYSIM_H
+#ifndef NBODYSIM_HPP
+#define NBODYSIM_HPP
 
 #include <string>
 
@@ -44,4 +44,4 @@ private:
     double meanForceMagnitude_;
 };
 
-#endif /* NBODYSIM_H */
\ No newline at end of file
+#endif /* NBODYSIM_HPP */
\ No newline at end of file