Explain grouping and summarizing in vignette?

[ewallace]

Vignettes currently give examples of taking median or mean of technical replicates, but don't really explain it. Since this is a core capability of dplyr, using group_by and summarise, we don't need a separate function or help section. But we do need to tell users how it works and point them to generic dplyr/tidyverse help.

[ewallace]

Our updated vignettes do a better job of explaining sample_id as the unique sample identifier. However we're not sure whether they do a good enough job introducing the split/apply/combine data analysis paradigm as in tidyverse group_by / summarise. We will ask questions about this in user interviews, and use that to decide on an action plan.