unnesting using `unnest_dt` and `unnestv_dt` are not flexible

[asiripanich]

Keeping for later..

These functions are currently in the mlogit branch. This is useful for creating a choice table for obtaining probabilities using a fitted choice model.

Currently these functions require all list columns to be unnested otherwise it won't work. This is not the behaviour of tidyr::unnest.

[asiripanich]

waiting for this unnest feature to be officially implemented in data.table https://github.com/Rdatatable/data.table/pull/4156