Changed cell_type selector to horizontal_filter which is much easier to use, and shows selected cell_types.
Deleted unique=true for cell_type, since we added a unique constraint for cell-type, species and cell_subtype.  South complained in the migration that it should have been deleted before applying the previous unique constraint.  It may be necessary to reverse the 3 way constraint, apply this change, and then reapply the 3 way constraint..
Merge pull request #36 from UPDDI/Issue-32

Fix Issue #32