Create a generic_spct object with wavelengths from wavebands in a list.
waveband or list of waveband objects The waveband(s) determine
the wavelengths in variable
A generic.spectrum object, with columns w.length set to the
union of all boundaries and hinges defined in the waveband(s).
Different spectral data variables are set to zero and added making the
returned value compatible with classes derived from