Validity checks for nested set functions
were not taking into account that the enclosed
set function may be on a nest level that is
lower than the nest level of the enclosing set
function.
Fixed by :
 - propagating max_sum_func_level
up the enclosing set functions chain.
 - updating the max_sum_func_level of the 
   enclosing set function when the enclosed set
   function is aggregated above or on the same
   nest level of as the level of the enclosing 
   set function.
 - updating the max_arg_level of the enclosing
   set function on a reference that refers to
   an item above or on the same nest level
   as the level of the enclosing set function.
 - Treating both Item_field and Item_ref as possibly
   referencing items from outer nest levels.