Functions for VISIR spectroscopy optical model
They can be used in the following way: 1) Initialize the instrument-specific constants by calling visir_spc_optmod_init(). 2) Compute the wavelength-range by calling visir_spc_optmod_wlen(). 3) The dispersion is assumed to be linear, D = (wlmax-wlmin)/(npix-1), thus the wavelength on pixel i is wli = wlmin + i * D, i = 1, 2, ..., npix.
The below functionality is implemented according to "Parameters for setting the VISIR Spectrometer" rev. 2.4