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   Kalman Filter for pitch
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   Form:
   [x, p] = PitchEst( x, p, dT, iZ, q, r, u, z, zValid )
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   ------
   Inputs
   ------
   x               (2,1)  State [angle;rate]
   p               (2,2)  Covariance matrix
   dT              (1,1)  Time step
   iZ              (1,1)  Pitch axis inertia
   q               (2,2)  Plant covariance (continuous time)
   r               (1,1)  Input covariance
   u               (1,1)  Deterministic input
   z               (1,1)  Measurement
   zValid          (1,1)  Threshold for valid measurements

   -------
   Outputs
   -------
   x               (2,1)  State
   p               (2,2)  Covariance matrix

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