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   Draw field lines on a sphere.
   You pass it a function that outputs the vectors. The function must be of 
   the form

   fun( x(3,:), jD )

   x is generated internally from latitude, longitude and r.
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   Form:
   DrawFieldLines( r, n, fun, jD )
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   ------
   Inputs
   ------
   r           (1,1)      Radius
   n           (1,1)      Number of latitude and longitude points 
   fun         (1,:)      Name of function
   jD          (1,1)      Julian date (days)

   -------
   Outputs
   -------
   None

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