Moved make-acme-node-vec code into make-acme-nn-stru

I want the intermediate distinct'ed flattened sequence
of map pairs to be available inside make-acme-nn-stru so
that I can add a field to the nn-stru that will map pairs
(sets of two) ids to indexes.
Add dummy :indexes-by-two-ids to make-acme-nn-stru

Done with call to currently dummy function make-index-by-two-ids-map,
which needs to be coded.  Right now it just returns a dummy map.