DiffOperation deprecated

Diff operation between two catalogues.

The name of 'Diff' is misleading because the operation has nothing to do with diff:

intersection = source ∩ target = {x: x ∈ source ∧ x ∈ target} all = intersection ∪ (target ∖ intersection) = target new = all ∖ source = {x: x ∈ target ∧ x ∉ source} obsolete = source ∖ all = source ∖ target = {x: x ∈ source ∧ x ∉ target}