Case 2: Absorption Coefficient - Model Uncertainties (case 2).

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The aim of this case is to investigate the uncertainties in absorption
calculation when the usual standard input is used for each model.
Absorption coefficients should be computed for specified
atmospheric conditions and frequencies but letting the remaining
parameters (e.g. spectroscopic data, line shape) to be chosen by the
participants.

The calculations will be carried out for a set of target species which 
have the signature in the given frequency range.
A standard continua absorption used by each model will be added to 
the line absorption.  

Input - Output Files
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 Format:
 ========
  The input files are ASCII,in ARTS format. 
  They can easily be recognized by the extension 'aa'.
  The file can start with an arbitrary number of comment lines.  
  These lines starts with the hash symbol (#) 
  The first row after the comment lines give the number of matrices 
  in the array. After this follows, for each matrix, a row giving 
  the matrix size followed by the data in row order.
 

 Input:
 ======
-  KeySpecies_case2.txt -->> a text file which gives the set of the
   target species for which the absorption coefficients should be calculated.
 - atm_scenario_case2.aa -->> profiles of temperature,
   altitude, pressure, and trace gas volume mixing ratios. The data is
   display in ARTS format. See the header of the file for a more
   detailed description.
 - Freq_mono_case2.aa -->> frequency vector.
 - p_z_abs_case2.aa -->> a 2 column matrix giving the pressure and
   the altitude grid for the output of the absorption coefficients. 
   The first column contains the levels in pressure units [Pa] while the 
   second column contains the corresponding levels in geometric altitude 
   units [m]. 
 
Output: 
========   
  - absorption coefficients on the grid given in p_z_abs_case2.aa. The
  output is requested to be displayed in ARTS format. Each row of the
  data matrix contains the computed absorption coefficients for the
  given frequencies at one pressure/altitude value.