Pass string arguments by reference

src/cluster/np_cluster.cpp

@@ -31,7 +31,7 @@
 
 using namespace std;
 
-extern void cluster(string config_file, string data_file, string output_path);
+extern void cluster(const string& config_file, const string& data_file, const string& output_path);
 
 /*! \brief Main program entry point.
  *

src/sphere/np_sphere.cpp

@@ -30,7 +30,7 @@
 
 using namespace std;
 
-extern void sphere(string config_file, string data_file, string output_path);
+extern void sphere(const string& config_file, const string& data_file, const string& output_path);
 
 /*! \brief Main program entry point.
  *