Use derivativeAtParam and not velocityAtParam

plugins/hppwidgetsplugin/pathplayer.cc

@@ -271,7 +271,7 @@ namespace hpp {
       plugin_->client()->robot()->setCurrentConfig (config.in());
       if (velocity_) {
         config =
-          plugin_->client()->problem()->velocityAtParam ((CORBA::ULong)pathIndex()->value(),currentParam_);
+          plugin_->client()->problem()->derivativeAtParam ((CORBA::ULong)pathIndex()->value(),1,currentParam_);
         plugin_->client()->robot()->setCurrentVelocity (config.in());
       }
       gepetto::gui::MainWindow::instance()->requestApplyCurrentConfiguration();