[#585] Weaken `Dupable` constraint for `getField`

Description

Currently getField requires the entire datatype to be Dupable (this was the simplest implementation, but also too restrictive for the real needs), this MR resolves this neatly so that only the obtained field itself has to be dupable.

The major issue here is that applying getField on a datatype consisting of dupable and non-dupable elements is easy to make inefficient, and I also try to make sure that the developer does not get unnecessarily expensive getField call accidentally.

Related issue(s)

Resolves #585 (closed)

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