make follow-rotation mode an absolute value

instead of proportional to interaction sphere of anchor node.
can be overridden by a node if one so wishes.

include/openspace/scene/scenegraphnode.h

@@ -136,6 +136,8 @@ public:
     double reachFactor() const;
     double approachFactor() const;
 
+    double followRotationDistance() const;
+
     bool supportsDirectInteraction() const;
 
     SceneGraphNode* childNode(const std::string& id);
@@ -212,6 +214,7 @@ private:
     properties::DoubleProperty _screenSizeRadius;
     properties::FloatProperty _visibilityDistance;
     properties::BoolProperty _supportsDirectInteraction;
+    properties::DoubleProperty _followRotationDistance;
 
     // This variable is used for the rate-limiting of the screenspace positions (if they
     // are calculated when _computeScreenSpaceValues is true)