Scene.clear() needs selective clearing

this is regarding the Scene class which allows to draw various compas geometry classes onto rhino.
Currently it appears that the Scene.clear() method clears all the layers in rhino and not just the layer that has an update.

It would be ideal to have the option to just clear the layer or sublayer that is updated.

code to recreate explanation above. (add a rhino object to any rhino layer before you run this)

```
from compas.geomtry import Point
from compas.scene import Scene

points = Point[0,0,0]

scene = Scene()
scene.clear()
scene.add(Point, layer="geometry::boundaries")
scene.draw()
```

pseudo code for request, something like

`Scene.clear( layer="geometry::boundaries") `