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_absorptance_based returns TRUE if its argument is a
filter_spct object that contains spectral absorptance 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.