work with datastore

[Licini]

Trying to work with this PR: compas-dev/compas#1470 , and enable the scene switching like below (which works, but we need to test more complicated cases):

from compas_viewer.viewer import Viewer
from compas.scene import Scene
from compas.geometry import Box, Translation

scene = Scene()
box = Box()
scene.add(box, color=(255, 0, 0), name="Box1")
group = scene.add_group(name="My Group")
obj2 = scene.add(box, color=(0, 0, 255), name="Box2", parent=group)
obj2.transformation = Translation().from_vector([2, 0, 0])


jsonstring = scene.to_jsonstring(pretty=True)
print(jsonstring)

scene_loaded = Scene.from_jsonstring(jsonstring)

viewer = Viewer()
viewer.scene = scene_loaded

viewer.show()

There is one compromise I had to make, we have to give up ability of set u, v parameters at object level.
The reason it that they require access to underlying item , which now has to go through self.scene.datastore[GUID]. However self.scene is not available until __init__ is finished. I cannot think of a quick and clean way yet, other than remove them all together... let me know if you have better ideas