allow for expressing invariance rules and optimizations around them

In some cases I've written rules like this:

```
@gc.rule(['location.id'], 'location.location_months.location.id')
def location_location_months_location_id(id):
return id
```

Because this transformation is opaque to graphcore (it sees it as a python function) and so can't optimize across that boundary as in this case:

```
location.location_months.location.date_of_first_email = (location.location_months.location.id)
location.establishment_type.name, location.name, location.establishment_type.id, location.house_state, location.id = () Cardinality.many
```

these SQLQueries should be mergable, but since there is this python function inbetween, it can't.