Use boost::variant Python binding to bind JointModel and JointData

[jorisv]

JointModel and JointData specific attributes are not binded to Python. Only JointModelBase and JointDataBase are part of the Python binding.

Since boost::variant support had been added lately to eigenpy, we can now retrieve the specific JointModel and JointData stored in DataTpl and ModelTpl. We can then bind all joint specific attribute to be able to use them in Python.

We can also use eigenpy to manage GeometryObject::meshMaterial.