Fix DataField.dist to limit group by fields
Fix DataField.dist to limit group by fields

Django requires QuerySet.values() to be called to alter the fields used
by the GROUP BY statement in the resulting SQL.

Signed-off-by: Byron Ruth <e9d71f5ee7c92d6dc9e92ffdad17b8bd49418f98@devel.io>

murphyke/avocado,murphyke/avocado,murphyke/avocado,murphyke/avocado