InterpolateCircle:

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   Interpolates between x1 and x2 when x1 and x2 lie in a circle. When
   f = 0 x = x1, when f = 1 x = x2.
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   Form:
   x = InterpolateCircle( x1, x2, f )
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   ------
   Inputs
   ------
   x1            (:,1)    First quantity
   x2            (:,1)    Second quantity
   f             (1,:)    0 to 1

   -------
   Outputs
   -------
   x             (:,1)    Solution

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