AirshipCV:

--------------------------------------------------------------------------
   Compute the x-location of the center of volume of an airship hull.

   A double-ellipsoid geometry is assumed, where the front ellipsoid
   has length a1, and the rear ellipsoid has length a2.
--------------------------------------------------------------------------
   Form:
   xcv = AirshipCV( a1, a2 )
--------------------------------------------------------------------------

   ------
   Inputs
   ------
   a1          (1)       semi-major axis of front ellipsoid
   a2          (1)       semi-major axis of rear ellipsoid

   -------
   Outputs
   -------
   xcv         (1)       x-Location of hull center of volume
                             (measured back from nose)

--------------------------------------------------------------------------