The functor members cannot be references because the Regular_triangulation
might be templated by a traits class that is not simply a Kernel (that is
the case in the package Interpolation).
When that is the case, calling this->K::function_object() might be a call
to a constructor in the base traits class. Thus, the functor members
of Reg_traits_adaptors get initialized to temporaries, which is problematic
because these temporaries are at a much lower scope and thus are quickly
cleaned, leaving us with references to nothing in Reg_traits_adaptor.
Mael Rouxel-Labbé
ac63b66286