Optimal actions from `state_space`

[DillonZChen]

For learning policies, it would be nice if the state space feature also extracted the optimal actions for each state in a problem on top of the optimal cost to go. Although it is possible to extract optimal actions from the optimal cost values, this takes a bit of code to do.

[drexlerd]

The state space generator will soon be deprecated. We are currently working on a complete rework of Mimir which will also allow more control about state space expansion. I can recommend using pymimir from its main branch for state space exploration. Note that it supports a smaller PDDL fragment. The future version in the dynamic branch will support PDDL 3.1. with the exception of object fluents.