Get delta states on Trace/TraceList, or in Extract

[e-cal]

For now I will put the function to get delta states in Extract, but it could also make sense to put the method on Trace and a wrapper function on TraceList.

Also would we prefer the function to take a single action and get all the delta states for the one action, or to always get all delta states for all actions in a Trace / TraceList?

[haz]

An interesting question...for now, let's put it on the State object, and if that's in an observation (like the identity observation), then you'll be able to use it in extract. So it'd be state.diff_from(other_state) (I think just using diff would be ambiguous).

[e-cal]

Sounds good!