RelativeDisturb

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  Relative disturbances for use with a relative state.
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  Form:
  aDiff = RelativeDisturb( jD, r, v, dr, dA )
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  Inputs:
  -------
  jD        (1)   Epoch (Julian date)
  r        (3,1)  Reference position in ECI frame (km)
  v        (3,1)  Reference velocity in ECI frame (km/s)
  dr       (3,1)  Relative position between the spacecraft (km)
  dA        (1)   Differential area between the spacecraft (m2)

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  Outputs:
  --------
  aDiff     (3,1)  sum of all the terms (m/s/s)

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