Make DocumentationEngine into a singleton object

Add documentation to DocumentationEngine
Add unique identifier to Documentation class (and enforce in DocumentationEngine)

include/openspace/documentation/documentationengine.h

@@ -27,24 +27,55 @@
 
 #include <openspace/documentation/documentation.h>
 
+#include <ghoul/designpattern/singleton.h>
+#include <ghoul/misc/exception.h>
+
 namespace openspace {
 namespace documentation {
 
-class DocumentationEngine {
+/**
+ * The DocumentationEngine has the ability to collect all Documentation%s that are
+ * produced in the application an write them out as a documentation file for human
+ * consumption.
+ */
+class DocumentationEngine : public ghoul::Singleton<DocumentationEngine> {
 public:
+    struct DuplicateDocumentationException : public ghoul::RuntimeError {
+        DuplicateDocumentationException(Documentation documentation);
 
+        Documentation documentation;
+    };
+
+    /**
+     * Write the collected Documentation%s to disk at the \p filename in the specified
+     * \p type. A new file is created and silently overwritten in the location that
+     * \p filename is pointed to.
+     * \param filename The file that is to be created containing all the Documentation 
+     * information.
+     * \param type The type of documentation that is written. Currently allowed values are
+     * \c text and \c html
+     */
     void writeDocumentation(const std::string& filename, const std::string& type);
 
-    void addDocumentation(Documentation doc);
+    /**
+     * Adds the \p documentation to the list of Documentation%s that are written to a 
+     * documentation file with the writeDocumentation method.
+     * \param documentation The Documentation object that is to be stored for later use
+     * \throws DuplicateDocumentationException If the \p documentation did not have a
+     * unique identifier
+     */
+    void addDocumentation(Documentation documentation);
+
+    static DocumentationEngine& ref();
 
 private:
+    /// The list of all Documentation%s that are stored by the DocumentationEngine
     std::vector<Documentation> _documentations;
-
 };
 
 } // namespace documentation
 } // namespace openspace
 
-
+#define DocEng (openspace::documentation::DocumentationEngine::ref())
 
 #endif // __DOCUMENTATIONENGINE_H__