Improve CmdFeature base class

Add default implementation of isEnabled() and add const

ApplicationLibCode/Commands/RicImportEnsembleFeature.h

@@ -35,7 +35,6 @@ class RicImportEnsembleFeature : public caf::CmdFeature
     CAF_CMD_HEADER_INIT;
 
 protected:
-    bool isCommandEnabled() override;
     void onActionTriggered( bool isChecked ) override;
     void setupActionLook( QAction* actionToSetup ) override;