implement extract in improper harmonic

src/MOLECULE/improper_harmonic.cpp

@@ -276,3 +276,15 @@ void ImproperHarmonic::write_data(FILE *fp)
   for (int i = 1; i <= atom->nimpropertypes; i++)
     fprintf(fp, "%d %g %g\n", i, k[i], RAD2DEG * chi[i]);
 }
+
+/* ----------------------------------------------------------------------
+   return ptr to internal members upon request
+------------------------------------------------------------------------ */
+
+void *ImproperHarmonic::extract(const char *str, int &dim)
+{
+  dim = 1;
+  if (strcmp(str, "k") == 0) return (void *) k;
+  if (strcmp(str, "chi") == 0) return (void *) chi;
+  return nullptr;
+}