Added battery state sensor to robot (requires an up-to-date strands-project/morse!)

Added battery sensor to the robot and a docking station and label to the bham cs_lg enviroment. The robot can now charge its battery when it drives onto the docking station. The autonomous docking is almost working but needs some final tweaks. 
Please note that the battery state is only useful for planning as it has no influence on the robot control.