Minor changes to functionality. #70

Add a config switch for chase car heading. Speed can be defaulted on but heading cannot.

The display reports vertical velocity in 2 places. The code computes sea_level_rate and uses it for the time to landing. I have found that use of sea level rate is very useful in determining parachute fouling. For instance you may have the predictor defaulted to 4m/sec. A computed sea level rate of 3 m/sec is a very clear signal the landing will be long. This is near constant whereas the time to landing is never constant and comparable to anything. Likewise the actual rate is always changing with altitude.