Added missing CanComputes in GeneralizedMorphismCategory.gi
Example composition of generalized morphisms

2 examples with nontrivial SourceAid and nontrivial RangeAid
for both generalized morphisms
ToDoList for GeneralizedMorphismObject

for IsWellDefined
Started serre quotient categories.
Merge remote-tracking branch 'Sepp/master'

* Sepp/master:
  ToDoList for GeneralizedMorphismObject
  Example composition of generalized morphisms
  Added missing CanComputes in GeneralizedMorphismCategory.gi