Movement is not first-come first-serve

All organisms give their movement "intentions".  These are then
analysed:
* Organisms wanting to move into occupied spaces take their turn after
the occupant.  If occupant doesn't move, they do not move either.  If
the space becomes free they do move.
* If >1 organism want to move to the same unoccupied space, a dice roll
(to be weighted on their health in future) determines who gets to move.
The rest have to stay where they are.