toMatrix: Converts 3d array of CATA data to a tall 2d matrix format
Description
Converts a three-dimensional array (\(I\) assessors, \(J\)
products, \(M\) attributes) to a two-dimensional matrix with
(\(I\) assessors, \(J\) products) rows and (\(M\)
attributes) columns, optionally preceded by two columns of row headers.