Adding option for constant velocity flight (closes #2150)

Using adapted code from James H

include/openspace/navigation/orbitalnavigator.h

@@ -196,6 +196,8 @@ private:
     properties::FloatProperty _stereoscopicDepthOfFocusSurface;
     properties::FloatProperty _staticViewScaleExponent;
 
+    properties::BoolProperty _constantVelocityFlight;
+
     properties::FloatProperty _retargetInterpolationTime;
     properties::FloatProperty _stereoInterpolationTime;
     properties::FloatProperty _followRotationInterpolationTime;