Heartrate zones

Due to another bugfix I was working on the workout graph and so added heart-rate zones

It might get rather philosophical on what heart rate ranges the different zones are defined. I decided on the following:

In green the Energy Efficient or Recovery Zone, up to 60% of you maximum heart-rate to develop a basic endurance and aerobic capacity.
In orange the Aerobic Zone, 60% up to 75% of your maximum heart-rate. Will develop your cardiovascular system.
In red the Anaerobic Zone, 85% up to 90% of your maximum heart-rate. Training in this zone will develop your lactic acid system.