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   Compute the solar force on a set of elemental areas.
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   Form:
   f = SolarF( p, rho, normal, source, area )
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   ------
   Inputs
   ------
   p             (1,1) Solar pressure
   rho           (3,n) Coefficient fraction, may also be (3,1)
                       [absorbed;specular;diffuse]
   normal        (3,n) Surface normal vector
   source        (3,1) Unit vector to source
   area          (n)   Elemental area

   -------
   Outputs
   -------
   f             (3,n) Solar force

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	  Reference: Hughes, P. C., "Spacecraft Attitude Dynamics"
              John Wiley and Sons, 1986, pp. 260-263.
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