If the organization of x is "vector", then x is a vector
representing a single spectrum. Compare this with ts. If the organization of x is "matrix", then x is a matrix
and the spectra are stored in the columns.
If the organization of x is "df.col", then x is a data.frame
with n+1 columns, where n is the number of spectra.
The wavelengths are stored in column 1, and the spectra in columns 2:(n+1).
This organization is good for printing to the console, and writing to files.
If the organization of x is "df.row", then x is a data.frame
with n rows, where n is the number of spectra.
The spectra are stored in the last column, which is a "model.matrix" with the name "spectra".
The other columns preceding spectra (if present) contain additional data associated with
the spectra; see extradata.