[route parsing] kill global_routing->size_of_link

[route parsing] kill global_routing->size_of_link

- Actually, all links we use are pointers to structure (how
  surprising). This strange thing survived until now because the
  constant model declared to have a size of links being the size of a
  double. But actually, since there is no link in constant routing, we
  could use 0 as size of links in that case.
- Also introduce a sg_routing_link_t that is simply an alias to void*
  for now. It is intended to represent what is stored into link_lib at
  the surf level (that is the core-most link concept)