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   Simplified atmosphere model. Agrees with the standard
   atmosphere model but is faster.
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   Form:
   d = SimpAtm( altitude, units )
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   Inputs
   ------
   altitude   (:)     Altitude (m or ft)
   units      (1,:)   'si' or 'eng' i.e. m & kg or lb & ft

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   Outputs
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   d          (:)     Standard atmosphere
                      .temperature
                      .pressure
                      .density
                      .speedOfSound
                      .kinematicViscosity

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