ir_stack: Stacks a matrix or data frame with spectra into a list column
Description
ir_stack takes a matrix or data frame with infrared spectra and
converts it into a list column corresponding to the column spectra in
objects of class ir.
A matrix or data frame with a first column (x) containing "x
axis values" of the spectra (e.g. wavenumbers) and all remaining columns
containing intensity values of spectra.