Add volume slice plugin

[superstar54]

from ase.build import molecule
from weas_widget import WeasWidget
from ase.io.cube import read_cube_data
volume, atoms = read_cube_data("h2o-homo.cube")
viewer = WeasWidget()
viewer.from_ase(atoms)
viewer.avr.model_style = 1
viewer.avr.volume_slice.volumetric_data = {"values": volume}
viewer.avr.volume_slice.settings = {"Slice 1": {"h": 0, "k": 1, "l": 0, "distance": 5.5, "samplingDistance": 0.1 },
                                    "Slice 2": {"h": 1, "k": 1, "l": 0, "distance": 5.5, "samplingDistance": 0.1 },
                                   }
viewer.camera.setting = {"direction": [0.5, 1, 2], "zoom": 1.5}
viewer