WPI: fix sgrid_traits compatibilty with google associative containers
WPI: fix sgrid_traits compatibilty with google associative containers

Storing reference to sgrid_handle  doen't work with google btree associative
containers, because they construct elements using empty ctor +
assignment operator.
That leads to mesh handle uninitialized in most iterators
so we actually have to store a COPY of handle in every sgrid_traits.

bs-eagle/bs-eagle,bs-eagle/bs-eagle,bs-eagle/bs-eagle