Add notifier function to allow hwinterface switching

This PR is motivated by the feature proposed in [gazebo_ros_pkgs#256](https://github.com/ros-simulation/gazebo_ros_pkgs/pull/256)
For more details see [gazebo_ros_pkgs#256](https://github.com/ros-simulation/gazebo_ros_pkgs/pull/256)

#184 adds a notification function that is called in ```ControllerManager::switchController```. This function can be used to switch the HardwareInterface for the resources of the controller that is meant to be started, i.e. **"Switch HardwareInterfaces on Switching Controllers.**

Default behaviour is simply ```return true``` - no effect.
Should you also have
```
stop_request_.clear();
start_request_.clear();
``` 
similar to the locations above?