Split datamodel into data and model

Datamodel is not really an explict name...
So I split vpltk.datamodel into vpltk.data and vpltk.model.

Main changes are:

vpltk.datamodel.data -> vpltk.data
vpltk.datamodel.model -> vpltk.model
vpltk.datamodel.python -> vpltk.model
I think it is better to move PythonModel code in python module inside "model" package