* Methods for Spectra objects
nrow returns the number of individuals in the collection
length returns the number of wavelengths in the collection
ncol returns NULL dim returns a vector containing (1) the
number of individuals and (2) in the number of wavelengths in the collection
* Methods for Spectra objects
nrow returns the number of individuals in the collection
length returns the number of wavelengths in the collection
ncol returns the number of attributes in the collection dim
returns a vector containing (1) the number of individuals, (2) in the number
of wavelengths, and (3) the number of attributes in the collection