Changes

tsk72 Fixes the handling of the favourites of the User class
The management of the favourites of a user has been improved by removing
the unused setter of the favourites list and adding the function to
check whether a story is in the favourites list or not. This function
allows to check when adding a favourite if a story was already a
favourite or not. On the other hand, it has been checked that the
stories to be added or deleted are not null, in case they are, an
exception will be thrown. Finally, the tests concerning the favourites
have been correctly implemented, since the existing ones did not check
their functionality, and the respective tests of the previously
commented corrections of the list of favourites have been added.