Better structure for transient ocean current plugin

[woensug-choi]

The current plugin requires a plugin to be loaded at the model description UDRF to switch on stratified ocean currents. It's because the stratified ocean current calculation requires a vehicle depth. A better way for further developments would be 'define all stratified parameters at the world file'. This can be brought up using gazebo's generic function called /gazebo/get_model_state.