Update output helper functions for nested data.tables?

[pratikunterwegs]

This issue requests that functions for epidemic size and new infections calculations should work on the output of vectorised model functions being added in #176. The return type of these helper functions is open for discussion - one straightforward option is to return the same type as the input, i.e., a data.frame/data.table for similar input, and a nested data.table with parameter set and scenario identifiers for similar input.

An alternative approach is to keep existing functionality (single-run, data.frame-like input), and show users how to run this function over a list column in a nested data.table/tibble.

[pratikunterwegs]

Superseded by outcomes_averted() in #225 and #230.