Documentation XWithGivenY

Explained the structure of UniversalObjects.gd
WithGiven method corrections
Merge branch 'master' of https://github.com/sebasguts/CategoriesForHomalg

* 'master' of https://github.com/sebasguts/CategoriesForHomalg:
  Inverse. Next step: Propagate knowledge.
  Source and range for dummy.
  Lazy category.
  Product of one object is another object.
Merge branch 'master' of https://github.com/sebasguts/CategoriesForHomalg

* 'master' of https://github.com/sebasguts/CategoriesForHomalg:
  Some add methods.