add function to transform sample output_type data to quantile

[nickreich]

A common task to support plotting of sample output_type data could be to transform that data to quantile-format and then plot, so that medians/intervals could be plotted. This is a simple transformation task, given a hub_df with sample data, turn it into quantiles.

[nickreich]

Noting that some care might need to be taken (and/or assumptions made) to impute quantiles that are not directly translatable from samples. e.g. what if you request a 2.5th and 97.5th quantile but only have 100 samples. Take the midpoint between the 97th and 98th ordered samples?

[elray1]

r.e. assumptions required to get quantiles, I suggest using R's built-in stats::quantile function, which has several options for how to handle this, specified by the type argument. The function we write in this package could also have a type argument which would then be passed to stats::quantile.

[elray1]

see also issue #11

[elray1]

I am proposing that this be an internal (i.e., not exported) function, with the user-facing function being a general-purpose function for converting between output types as in #11.