 lbl2xs
  computation of line-by-line molecular absorption cross sections

  usage:
  lbl2xs [options] 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)

  -p  float(s)  pressure(s) (in mb,  default: p_ref of linefile, usually 1013.25mb=1atm)
  -T  float(s)  Temperature(s) (in K, default: T_ref of linefile, usually 296K)
 --pT file      with list of pressures in mb (first column) and temperatures in K (second column)

  -n  int       number of grids --- selects 'multigrid mode' for nGrids=2 or 3  (default: nGrids=1 brute force)
  -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)

  -f  string    format for output file: 'a'='t'='xy' tabular ascii OR 'h' hitran OR pickled (default) 
  -i  string    interpolation method   [default: '3' 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]
  -m  string    molecule (no default, should be given in the line file, required otherwise)
  -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)
  -x  Interval  lower,upper wavenumbers (comma separated pair of floats [no blanks!],
                                        default set according to range of lines in datafile)


  If several line parameter files are given (usually for several molecules)
  AND if an output file (extension) has been specified (-o option)
  a cross section file will be generated for each line file:
  * if all line files have the same extension, the cross section files will have the old basename with the extension as specified by the -o option
  * otherwise the input file name will be augmented by the string specified as -o option
  
  The line parameter file should contain a header section indicating the molecule, pressure, temperature,
  and type of columns followed by a list of (preselected) lines in the format
  # molecule: XYZ
  # temperature: 296 K
  # pressure:    1013.25 mb
  # format: vSEan
  position1  strength1  energy1  airWidth1  tDep1
  position2  strength2  energy2  airWidth2  tDep2
  position3  strength3  energy3  airWidth3  tDep3
  .........  .........  .......  .........  .....

  This file can be generated from HITRAN, SAO, or GEISA database with extract.py 
  The line list should contain at least 2 columns with line positions (in cm-1) and line strengths.
  If lower state energy is missing, cross sections can be calculated only at the reference temperature of the line list.
  If air broadened half width is missing, it can be set to a (constant) default value with the -a option.
  If temperature dependence of the air broadened half width is missing, the value in the molecular data file is used.

