Suggestion: Add support for exploded views to assembly

As far as I can tell, assemblies don't yet support exploded views.

One way to support this would be to add an `explode` vector parameter to the `add` function which was used to how the part moved when the assembly was exploded, eg

```
door = (
cq.Assembly()
.add(make_vslot(H), name="left", explode=(-50, 0, 0))
.add(make_vslot(H), name="right", explode=(50, 0, 0))
.add(make_vslot(W), name="top", explode=(0, 0, 50))
.add(make_vslot(W), name="bottom", explode=(0, 0, -50))
.add(make_connector(), name="con_tl", color=cq.Color("black"))
...
```