[Feat] Add color

[hanson-hschang]

• Add update_material method

  • only has the ability to update color, currently

• Add class Light in _light.py to control light position

• Add rendering feature in the _camera.py to render images from the camera point of view

• Create a simple example to use the update_material and to update color. Integrate with the visualization of the director Visualize director #13.

• Some examples have bug and therefore can't run properly. Create an issue for it: Fix bugs in all examples #31.

• A question on the protocol of the update_states method. Create an issue for it: Protocol on update_states #32.- [ ]