unassign operation

[matt-beanland]

enhance the assigner to support the unassign operation

this needs to delete the relationship characteristic and then the relationship

[matt-beanland]

cannot delete a relationship with a characteristic, seems to be guarded both ways.
characteristic has guard on instance, feature and relationship
instance doesn't have a guard on characteristic
feature doesn't have a guard on characteristic - deleting the feature leaves the characteristic.

We really should be able to remove a characteristic from a feature or a relationship.

[matt-beanland]

unrelate_relationship_characteristics simply unrelates the characteristics, we possibly want to remove them?
unrelate_feature_characteristics simply unrelates the feature characteristics, we possibly want to remove them?

possibly add a new action that deletes? Tried cascade_destroy but it left characteristics.

Updated the assigner so that it unrelates the relationship characteristic before removing them and the relationship.