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
I wonder if it is worth asserting that ```coord._points``` was a lazy array before doing the operation. This is for two reasons:

 1. You are actually checking that LazyArray aggregate_by is taking place
 2. When we remove LazyArray, the test will fail (and we can link to this PR/issue to revert back to using ``_points``)