cleanup Kalman filter parameters, add/fix comments/units

// -----------------------------------------------------------------------

static const float gyroSTDEV = 3.16e-4; // rad/s^3/2

/*

 * gyroVAR gives the measured variance of the gyro's output per

 * Hz (or variance at 1 Hz). This is an "intrinsic" parameter of the gyro,

 * which is independent of the sampling frequency.

 *

 * The variance of gyro's output at a given sampling period can be

 * calculated as:

 *      variance(T) = gyroVAR / T

 *

 * The variance of the INTEGRATED OUTPUT at a given sampling period can be

 * calculated as:

 *       variance_integrate_output(T) = gyroVAR * T

 *

 */

static const float gyroVAR = 1e-7;      // (rad/s)^2 / Hz

static const float biasVAR = 1e-8;      // (rad/s)^2 / s (guessed)

/*

 * Standard deviations of accelerometer and magnetometer

 */

static const float accSTDEV  = 0.05f;   // m/s^2 (measured 0.08 / CDD 0.05)

static const float magSTDEV  = 0.5f;    // uT    (measured 0.7  / CDD 0.5)

static const float biasSTDEV = 3.16e-5; // rad/s^1/2 (guessed)

static const float FREE_FALL_THRESHOLD = 0.981f;

    x0 = q;

    x1 = 0;

    // process noise covariance matrix

    //  G = | -1 0 |

    //      |  0 1 |

    const float v = gyroSTDEV;

    const float u = biasSTDEV;

    const float q00 = v*v*dT + 0.33333f*(dT*dT*dT)*u*u;

    const float q10 =              0.5f*(dT*dT)   *u*u;

    // process noise covariance matrix: G.Q.Gt, with

    //

    //  G = | -1 0 |        Q = | q00 q10 |

    //      |  0 1 |            | q01 q11 |

    //

    // q00 = sv^2.dt + 1/3.su^2.dt^3

    // q10 = q01 = 1/2.su^2.dt^2

    // q11 = su^2.dt

    //

    // variance of integrated output at 1/dT Hz

    // (random drift)

    const float q00 = gyroVAR * dT;

    // variance of drift rate ramp

    const float q11 = biasVAR * dT;

    const float u   = q11 / dT;

    const float q10 = 0.5f*u*dT*dT;

    const float q01 = q10;

    const float q11 = u*u*dT;

    GQGt[0][0] =  q00;

    GQGt[0][0] =  q00;      // rad^2

    GQGt[1][0] = -q10;

    GQGt[0][1] = -q01;

    GQGt[1][1] =  q11;

    GQGt[1][1] =  q11;      // (rad/s)^2

    // initial covariance: Var{ x(t0) }

    // TODO: initialize P correctly

    P = 0;

}