teach pair potentials to open potential files via open_potential()

src/USER-MISC/pair_tersoff_table.cpp

@@ -857,7 +857,7 @@ void PairTersoffTable::read_file(char *file)
 
   FILE *fp;
   if (comm->me == 0) {
-    fp = fopen(file,"r");
+    fp = open_potential(file);
     if (fp == NULL) {
       char str[128];
       sprintf(str,"Cannot open Tersoff potential file %s",file);