Fix aggregate_by bug

Occasionally `aggregate_by` will fail when aggregating on an `AuxCoord`, because the `_points` attribute can be of type `iris.aux_factory.LazyArray`. This PR fixes this by forcing the grouping to be done on the `points` attribute instead.

Longer-term it would be better if the `LazyArray`s were replaced with Biggus arrays, but one step at a time.

Fixes #1119