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/documentation/documentationgenerator.h

@@ -82,7 +82,6 @@ public:
      */
     void writeDocumentation(const std::string& filename);
 
-protected:
     /**
      * This abstract method is used by concrete subclasses to provide the actual data that
      * is used in the documentation written by this DocumentationGenerator class. The JSON