Introduce ModelData class

[samaneh-kazemi]

Reference Issue

The goal of this PR is to split ModelScene into ModelData and SceneData class. First sub task is to introduce ModelData class, ModelScene class will have a memeber of this type. I gradually start adding model related members of ModelScene to ModelData class. Making sure the functionalities still work and maybe add unit tests for it.

Eventually once we introduce extra-model element, ModelScene will have an array of ModelData per each extra model