is_absorbance_based returns TRUE if its argument is a filter_spct
object that contains spectral absorbance data and FALSE if it does not contain
such data, but returns NA for any other R object, including those belonging
other generic_spct-derived classes.
is_transmittance_based returns TRUE if its argument is a filter_spct
object that contains spectral transmittance data and FALSE if it does not contain
such data, but returns NA for any other R object, including those belonging
other generic_spct-derived classes.