Update Doxygen (#2537)

* Modernizing Doxygen
* Remove warnings for Doxygen

include/openspace/engine/moduleengine.h

@@ -45,7 +45,7 @@ namespace scripting { struct LuaLibrary; }
  * The ModuleEngine is the central repository for registering and accessing
  * OpenSpaceModule for the current application run. By initializing (#initialize) the
  * ModuleEngine, the default set of OpenSpaceModule%s as generated by CMake in the
- * <code>moduleregistration.h</code> file is automatically registered and created.
+ * `moduleregistration.h` file is automatically registered and created.
  * Additional OpenSpaceModule%s can be registered with the #registerModule function, which
  * will internally call the OpenSpaceModule::initialize method.
  */
@@ -55,7 +55,7 @@ public:
 
     /**
      * Registers all of the OpenSpaceModule%s which are created by the CMake configuration
-     * and stored in the <code>moduleregistration.h</code> file. For all of those modules
+     * and stored in the `moduleregistration.h` file. For all of those modules
      * the OpenSpaceModule::initialize method with will called.
      *
      * \throw ghoul::RuntimeError If two modules in the default modules have the same
@@ -101,7 +101,7 @@ public:
 
     /**
      * Get the module subclass with given template argument. Requires the module subclass
-     * to have the public static member variable <code>name</code> which must be equal to
+     * to have the public static member variable `name` which must be equal to
      * the name of the module used in its constructor.
      *
      * \return a pointer to the module of the given subclass