 cross_section

  Read and write / plot molecular cross sections (e.g., to reformat or interpolate).

  usage:
  cross_section [options] xs_file(s)

  command line options:
   -c   char(s)  comment character in input file(s) (default #)
   -C   ints     sequence of integers: cross section (levels/layers) to extract
                 (in the xy ascii file 0 corresponds to the first column=wavenumber)
   -f   string   format for output file [default: 'xy', 'h' for hitran format]
   -h            help
   -i   string   interpolation method for spectral domain (cross section vs wavenumber)
                 "2", "3", "4" for Lagrange interpolation, "s" for spline
                 default: '3' three-point Lagrange
		 '0' in combination with 'xy' tabular output format generates individual files for each p, T, molecule
  --plot         matplotlib for quicklook of cross sections
   -o   file     output file (default: standard output)

  Cross Section Files:
  *   xy formatted ascii file with wavenumbers in column 1 and cross section(s) (for some p,T) in the following column(s).
  *   hitran formatted cross section file
  *   pickled cross section file (default output of lbl2xs)

  NOTE:  
  *  If no output file is specified, only a summary 'statistics' is given !!!
  *  xy tabular output format:
     For each molecule cross sections of all p,T pairs will be interpolated to a common wavenumber grid and saved as 'matrix';
     To write an individual file for each p/T, suppress interpolation with '-i0' option.
  *  When reading files,  cross_section tries to determine the format from the first line (record)
  *  When reading Hitran xs files, the header record is sometimes incorrectly formatted (missing blanks), and cross_section might fail.
     (Reading files with xs data distributed over several records is not yet implemented!).

