improve representative frame getter

[keighrim]

can we provide an argument to set the rule to pick the "top"? I'm thinking of

1. get all fnums from all elements in the rep list.
2. get the fnum of the first rep element (as a single list maybe, only to keep the return type simple)
3. get the fnum of the "middle" rep element

We might need to think more carefully how this argument can be propagated in the following extract_representative_frame wrapper fn.

Originally posted by @keighrim in #278 (comment)

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