Deleted RestrictedGeneralizedMorphismCategory

and installed new IsWellDefined method for GeneralizedMorphisms
Deleted ToDo-management from GeneralizedMorphismCategory
IdentityMorphism in GeneralizedMorphismCategory
Renamed: AssociatedMorphism -> MorphismAid

because the associated morphism is part of the triple
(domain, associated morphism, codomain)
and not part of the triple
(source aid, morphism aid, range aid).