HRakoczy:

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   Height function for a square sail with billow.
   The deflection increases radially from the center to the corners.
   The center of the sail is at h = 0.

   Since version 7.
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   Form:
   h = HRakoczy( x, y, d )
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   ------
   Inputs
   ------
   x       (1,m)      x position
   y       (1,m)      y position
   d        (:)       Data structure
                      .p   (1,1)   Percentage billow (ex. 0.01)
                      .L   (1,1)   Sail diagonal length (m)

   -------
   Outputs
   -------
   h       (1,m)      Out-of-plane position of points

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   Reference: John Rakoczy, NASA MSFC
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