cf.aggregate fails when field units are non-valid

[davidhassell]

At v3.10.0, if otherwise aggregatable fields have non-valid units (such as kg -m-2 s-, cf.aggregate fails, ultimately when cf.Data.concatenate refuses to aggregate data with non-equivalent units. Since cfunits v3.3.1, non-valid units are no longer equivalent.

In this case, cf.aggregate needs to return, unaggregated, the fields with bad units, and report that it has done so.