Implemented Pullback
convenience methods for KernelLift/ CokernelColift
Merge branch 'master' of momo.mathb.rwth-aachen.de:/home/gutsche/gap/repos.git/CategoriesForHomalg

* 'master' of momo.mathb.rwth-aachen.de:/home/gutsche/gap/repos.git/CategoriesForHomalg:
  Fixed bug in equality of morphisms.
Added requirements for UniversalMorphismIntoPullbackWithGivenPullback
Started generalized morphism category.