provide template samplesheet for de

right now possibly the hardest bit for a user trying to work the de workflow is putting together a samplesheet and classifying columns for it. if we provide a template ppl can fill, then its not only clearer what the req input is but it also gives ppl a head start making it. so i think the proposal here would be:

in the workflow stepper step where we ask for samplesheet upload, provide somewhere obvious a way to download a template samplesheet w basic structure. i think thatd mean mandatory cols are there, some ex optional ones maybe also there w clear indication they can be swapped for other similar things

Start at the workflow stepper step where users upload a samplesheet and trace how its required columns are defined. Add an obvious way to download a template containing the mandatory columns and clearly marked optional or replaceable columns. Done means users can download and fill the template before uploading it.

Written by the indexing model from the issue text.