Restore possibility of having neither RPA nor IPA in ResponseCmd

The choice of having neither RPA nor IPA corresponds to full response.
It was incorrectly removed.

src/ResponseCmd.C

@@ -98,12 +98,6 @@ int ResponseCmd::action(int argc, char **argv)
         cout << " Only one of -RPA or -IPA can be specified" << endl;
       return 1;
     }
-    if ( !rpa && !ipa )
-    {
-      if ( ui->onpe0() )
-        cout << " Select -RPA or -IPA" << endl;
-      return 1;
-    }
 
     double amplitude = 0.0;
     if ( !strcmp(argv[iarg],"-amplitude") )

src/ResponseCmd.h

@@ -42,7 +42,7 @@ class ResponseCmd : public Cmd
     return
     "\n response\n\n"
     " syntax: response amplitude nitscf [nite]\n"
-    "         response -vext vext_file [-cube] {-RPA|-IPA} [-amplitude a] \n"
+    "         response -vext vext_file [-cube] [-RPA|-IPA] [-amplitude a] \n"
     "                  nitscf [nite]\n\n"
     "   The response command computes the polarizability tensor by\n"
     "   finite differences using external electric fields in the x,y,z\n"