ENH: Update model_factory.hpp

Here #pragma once was missing and caused error while creating bindings so added it.

include/model_factory.hpp

@@ -1,3 +1,4 @@
+#pragma once
 #include "config_parser.hpp"
 #include "model.hpp"
 #include "models/ActivityDrivenModel.hpp"
@@ -114,4 +115,4 @@ create_model_deffuant_vector( Network<AgentT> & network, const ModelVariantT & m
     }
 }
 
-} // namespace Seldon::ModelFactory
+} // namespace Seldon::ModelFactory