[Dependencies] Update to Athena-Core 1.2.1
Add a function to deserialize an Entity from a JSON string

Including the management of delayed properties
Add a function to serialize a Scene into a rapidjson object
Add a function to deserialize a Scene from a rapidjson object
Add a function to serialize a Scene into a JSON string
Add a function to deserialize a Scene from a JSON string
Add a function to serialize a Scene into a DataStream object
Add a function to deserialize a Scene from a DataStream object