  lbl2od
  computation of line-by-line optical depth due to molecular absorption

  usage:
  lbl2od  [options]  atm_file  line_parameter_file(s)

  -h             help
  -c   char      comment character(s) used in input,output file (default '#')
  -o   string    output file for saving of cross sections (if not given: write to StdOut)
                 (in case of a list of input files this options specifies just the extension)

  -m   char      mode: 'c' ---> cumulative optical depth
                       'd' ---> difference (delta) optical depth (default)
                       'r' ---> reverse cumulative optical depth
		       anything else: total optical depth

  -i   string    interpolation method   [default: '2' three-point Lagrange]
                 (only required for multigrid approach or when cross sections for several p,T pairs are saved in xy format)
  -L   string    Lineshape: V(oigt), L(orentz), G(auss)     [default: Voigt]
  -q   char      quadrature method: default 'T'=trapez, 'S'=simpson, 'H' = piecewise cubic Hermite
  -s   float     sampling rate used for x-grid (default: 5.0 grid points per (mean) half width)
  -w   float     wing extension (cutoff wavenumber, default 10.0cm-1)

  -n   int       number of grids --- selects 'multigrid mode' for nGrids=2 or 3  (default: nGrids=2 two-grid)
  -g   int       gridRatio = ratio of coarse to fine grid spacing  (only 2, 4, or 8, default 8)
  -W   float     transition from fine to coarse grid  (in units of half widths, default 25.0)

 --ToA float     top-of-atmosphere altitude (compute opt.depth only for levels below) 
  -x   Interval  lower,upper wavenumbers (comma separated pair of floats [no blanks!],
                                          default set according to range of lines in datafiles)
 --nm            on output write optical depth versus wavelength [nm] (default: wavenumber 1/cm)
  -y             on output write only optical depth without x-values (wavenumbers etc)


  The line parameter file(s) should contain a list of (preselected) lines
  that can be generated from HITRAN or GEISA database with extract.py 
  (See the documentation header of lbl2xs.py for more details)

