Annotation extensions are not available for phenotypes

There is only one annotation extension relation for phenotypes (assayed_using)

it is implemented as assayed_using(PomBase gene)

This would be modelled the same as the "has_input" relation described for GO extensions here:
https://github.com/intermine/pombemine/issues/52

because it is the same type of relation (both represent the target of an entity, i.e the entity affected or acted upon)