update `etn_fields` vignette

[PietrH]

          > Peter once mentioned to me that we are no longer maintaining the etn_fields vignette

I made that comment in error. It is referenced from dataset metadata in IMIS:

while I see value in it, I believe making major changes to it to be out of scope of v2.3

Agreed

Originally posted by @peterdesmet in #306 (comment)

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the etn_fields vignette is currently out of date, and possibly broken. It should be updated.