[MAINTAIN] clear up confusion around last/prev/final

[sash-a]

Please describe what needs to be maintained?

In some of the code we call the previous step's observation/done last_done and in others we call it prev_done. Additionally, when calculating GAE we usually call it last_done when what we mean is "the last done of the rollout". I think we can clear up the ambiguity by always using prev to mean 1 step back and final to mean the final step of the rollout.

Describe the outcome you'd like

A clear and concise description of what you want to happen.

How do we know when maintenance is complete?

Checklist:

• the ambiguity around last is cleared up
• all systems use the same terminology