Fix bug in solving discounted ARAT games

The code made the assumption that the game was perfect information & in
moving through strategy changes in adj_improve_discount.
Don't restrict to adjacent strategies on each iteration

The function adj_improve_discount and adj_improve_average are renamed
without the leading "adj_".  The code in these two functions now match
in not restricting to adjacent improvements. Hence the name change.
Change function names to reflect non-adjacent checks

Rename adj_improve_discount and adj_improve_average without the leading
adj_ to reflect the fact that they aren't restricted to adjacent
strategies. Also delete multi_improve_average because it is a duplicate
of the new improve_average.