Table row access ergonomics

[molpopgen]

The current row access API returns Result<Option<_>, _>. The error state happens when inputs are out of bounds. We should take a clue here from stdlib and just return None for such out of bounds cases.

IMO, it is worth it to streamline code using this part of the API.

See also #327, which is the same issue for a different part of the API.

[molpopgen]

Closing this for now -- out of range being treated as an error is fine. Client code can convert error to option in the usual way.

[molpopgen]

Reopening -- it'd be nice to be more idiomatic w/respect to indexes not in range.

[molpopgen]

Closed by #351 and #355