Implement `Game.get_own_prior_actions`

[d-kad]

Implement GameTreeRep::GetOwnPriorActions (C++) and Game.get_own_prior_actions (Python) to compute, for a given information set, the set of last actions taken by the player acting in the information set before reaching it

Closes #582

[rahulsavani]

@rahulsavani Do you agree with my analysis, or again am I misunderstanding something?

This sounds correct to me, and I like that we can just return the empty list for unreachable info sets, and indeed this will be a characterisation of them, right?

[tturocy]

Note that the tests now failing is because I made a clarification change to the text of the exception which now doesn't match the test. However given the discussion we probably won't be raising that exception anymore.