PillarsDF: Make each pillar of a 3D array into a column of a tibble
Description
Create a data frame (tibble) where pillar [i, j, ] is column i + n1 *
(j - 1) with column name "i_j" (we use an underscore here rather than a
comma because a comma is the delimiter in csv files so writing this data
frame to a csv could cause confusion).