Feature/documentation topic (#822)

- Implement documentation topic that can be used to query documentation using the network API.
- Implement a way to pass arguments to lua scripts using json (rather than formatting entire lua string clientside)
- Implement ability to attach callback to lua script executions
- Implement abillity to transport return values from lua scripts back to network API clients.
- Do not initialize server interface on slave nodes.
- Implement Dictionary -> json converter using nlohmann json library

include/openspace/rendering/renderengine.h

@@ -53,8 +53,6 @@ class SceneManager;
 class ScreenLog;
 class ScreenSpaceRenderable;
 struct ShutdownInformation;
-class Syncable;
-class SyncBuffer;
 
 class RenderEngine : public properties::PropertyOwner {
 public: