Extent extract method of BuckCoulLong

src/KSPACE/pair_buck_coul_long.cpp

@@ -558,5 +558,8 @@ void *PairBuckCoulLong::extract(const char *str, int &dim)
 {
   dim = 0;
   if (strcmp(str,"cut_coul") == 0) return (void *) &cut_coul;
+  dim = 2;
+  if (strcmp(str,"a") == 0) return (void *) a;
+  if (strcmp(str,"c") == 0) return (void *) c;
   return nullptr;
 }