fix: Virtually inherit from IIdentifiable

Interface classes derived from IIdentifiable are prone to diamond inheritance.
Virtually inherit from IIdentifiable to ensure only one copy is inherited.

Closes #64

See merge request !118

include/MantleAPI/Traffic/i_controller.h

@@ -25,7 +25,7 @@
 namespace mantle_api
 {
 /// Base interface for all controllers.
-class IController : public IIdentifiable
+class IController : public virtual IIdentifiable
 {
 public:
   /// Desired state of lateral domain

include/MantleAPI/Traffic/i_entity.h

@@ -49,7 +49,7 @@ struct EntityVisibilityConfig
 };
 
 /// Base interface for all static and dynamic scenario entities.
-class IEntity : public IIdentifiable
+class IEntity : public virtual IIdentifiable
 {
 public:
   /// @brief Sets the position of the scenario entity