Set the wavelengths to be selected. To be used together with predefined wavelength grids, such as transmittance_wl_file, molecular_tau_file and particularly useful in combination with the correlated_k option where often only a specified number of wavelength bands is required. E.g., in combination with correlated_k AVHRR_KRATZ, wavelength_index 15 15 will select wavelength index 15 which corresponds to channel 4, or wavelength_index 10 14 will select those bands required for channel 3. Indices start from 1.