Address stability problems for camera paths and make them more useful in general

* Change spline curve parameter interval from [0,1] to [0,nSegments], for slightly increased precision
* Use linear paths whenever precision isn't enough/paths are too long (and traverse linear paths differently, to make them less depending on the distance)
* Remove OrbitalNavigator.LinearFlight and instead add "Zoom to" helper functions (closes #1837 )
* Add check to prevent triggering awkward short paths, for example when flying to the current target or to the same geo position twice
* Refactor speed code and handle speed when path is shorter than start+closeup distance
* Add easing to speed dampening
* Add topic for engine mode (to be used for UI later)
* Cleaner log when creating paths (some previous info messages are now debug messages)

Related PR: OpenSpace/OpenSpace-WebGuiFrontend#70

include/openspace/interaction/sessionrecording.h

@@ -671,7 +671,6 @@ protected:
     bool isPropertyAllowedForBaseline(const std::string& propString);
     unsigned int findIndexOfLastCameraKeyframeInTimeline();
     bool doesTimelineEntryContainCamera(unsigned int index) const;
-    std::vector<std::pair<CallbackHandle, StateChangeCallback>> _stateChangeCallbacks;
     bool doesStartWithSubstring(const std::string& s, const std::string& matchSubstr);
     void trimCommandsFromScriptIfFound(std::string& script);
     void replaceCommandsFromScriptIfFound(std::string& script);
@@ -800,6 +799,7 @@ protected:
     double _cameraFirstInTimeline_timestamp = 0;
 
     int _nextCallbackHandle = 0;
+    std::vector<std::pair<CallbackHandle, StateChangeCallback>> _stateChangeCallbacks;
 
     DataMode _conversionDataMode = DataMode::Binary;
     int _conversionLineNum = 1;