Add Entity::HasComponents() wrapper

src/entity_i.cpp

@@ -17,3 +17,7 @@ template <typename CompT> CompT *Entity::GetComponent() {
     return dynamic_cast<CompT *>(comp_ptr.get());
   }
 }
+
+template <typename CompT> bool Entity::HasComponent() {
+  return GetComponent<CompT>() != nullptr;
+}