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   Computes the emittance of plates in parallel.
   The heat flux through the plates is then sigma T^4/nB.
   The rest is reflected.

   Type EmittanceParallePlates 

   for a demo
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   Form:
   nB = EmittanceParallelPlates( e )
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   Inputs
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   e           (1,:)    Plate emittances

   -------
   Outputs
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   nB          (1,1)    Number of black plates

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