add function to transform sample output_type data to mean

[nickreich]

Similar to #8, we are looking for a simple function that can take sample output_type data and compute, for each individual prediction unit, a mean, and return that as a model_out_tbl.

[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.