Allow DSG tractories with identical trajectory_id values to be aggregated.

[davidhassell]

Currently you can't aggregate with cf.aggregate DSG trajectory fields with identical cf_role=trajectory_id coordinates, because of the general rule that when no 1-d coordinates for a given axis are different between fields, no aggregation can occur on that axis.

However, this is too restrictive for DSG trajectories, for which we may want to aggregate multiple realizations of the same trajectory path.