Make adding sensors easier

[mattpow35]

Adding sensors to agents programmatically should be easier for users. Right now you would have to create a sensor definition and pass it to env.agents['name'].add_sensors(SensorDefinition(params...)). This isn't too complicated, but is unintuitive for users. I think it might be better to avoid having the user see the sensor definition, and instead provide a simple function that takes all the params for a sensor def rather than the def itself and adds the sensor under the skin. Adding sensors (which includes all tasks) is common and should be simple and intuitive.