ENH: action values for zero-probability infosets should be undefined

[rahulsavani]

Currently, the action value (stored in map_actionsValues) for actions at zero-probability information sets are zero. This enhancement would instead have these recorded as undefined. This may require changing the type of values of the map which is currently, via a template, either a double of Gambit.Rational, or just not populating it for the respective actions at zero-prob infosets.

[tturocy]

We'll also need to give some consideration about the best way to do this, given that C++ does not have a natural "null" value. One possibility is to raise an exception, though this will take a bit of care to ensure code that calls this currently is ready for that.