IsZero for objects
Implemented IsMonomorphism and IsEpimorphism methods
IsIsomorphism method
example: VectorSpacesPullback
Bugfix: Codominates, Dominates recursion
Bugfix: Dominates, Codominates (swapped method implementations)
Bugfix: IsWellDefined propagation of attributes
Installed: EpiMonoFactorization
Implemented method to add special method.
Merge remote-tracking branch 'Sepp/master'

* Sepp/master:
  Installed: EpiMonoFactorization
  Bugfix: IsWellDefined propagation of attributes
  Bugfix: Dominates, Codominates (swapped method implementations)
  Bugfix: Codominates, Dominates recursion
  example: VectorSpacesPullback
  IsIsomorphism method