Add a single spectrum keyword to extract region

Add a single spectrum keyword to extract region: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). So I propose we add a keyword to extract region that allows it to return a single spectrum for the multi sub-region case. This could be just return single spectrum…

The task

So I propose we add a keyword to `extract_region` that allows it to return a *single* spectrum for the multi sub-region case. This could be just `return_single_spectrum` which would default to False but when True when trigger the discussion above. But that's kind of a long keyword name. Any other ideas?

Part of FineEnvs/MiMo-V2.6-RL-harbor-code.