Domain class methods for ForEachGeometryElementZone (e.g. .instance(geometry))

Other domain-bearing nodes expose the domain as class methods (`g.CaptureAttribute.face(...)`, `g.StoreNamedAttribute.point.float(...)`), but the for-each zone takes a keyword:

```python
zone = g.ForEachGeometryElementZone(pieces.o.instances, domain="INSTANCE")
```

Add domain class methods for consistency:

```python
zone = g.ForEachGeometryElementZone.instance(pieces.o.instances)
```
