Fix nit about distribution computation:
Fix nit about distribution computation:

Enable having two states evolving in the same state after the mean field node.

I find this issue by coding a MFG where the dynamics depend on the distribution.
My game is a mean field routing game where travel time on link depends on the distribution.
deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel