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   Outputs the set of acceptable attitude measurements from stars.
   Computes the measurement matrix, measurements and noise
   matrix given a pixel map from the unit vector measurements. Will update
   q if it has three valid vectors and the attitude error is large 
   See also U2Pix
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   Form:
   [h, dz, R] = Measurements( r, sensorData, qECIToB, qBToS, catalog, fScale )
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   Inputs
   ------
   r            (1,1)    Measurement noise
   sensorData   (:)      Structure
                          .xy    Pixel values in the xy focal plane
                          .valid 1 = valid measurement
   qECIToB      (4,1)    Quaternion from ECI to body axes
   qBToS        (4,1)    Quaternion from body to sensor axes
   catalog      (3,:)    Target catalog unit vectors
   fScale       (1,1)    Sensor scale factor

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   Outputs
   -------
   h            (n,6)    Linearized measurement matrix
   dz           (n)      The measurement residuals
   R            (n,n)    Measurement noise           
   qECIToB      (4,1)    Quaternion from ECI to body axes

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