improve performance + precision

[RobTillaart]

There are 3 hypot() calls in read(). These can be replaced by the equivalent

    float a2 = a * a;
    float b2 = b * b;
    float c2 = c * c;
    x = sqrt(a2 + b2);
    y = sqrt(b2 + c2);
    z = sqrt(c2 + a2);

advantage is a slightly 26 us faster read() on an UNO (163 us -> 137 us) as squares are only calculated once, and smaller size ~258 bytes ~ (number based upon test sketch).

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use micros() for the duration to correct pitch yaw etc.
NB with a throttle of 10 millis the accuracy of millis is typically in 2 decimals.
By using micros() one would expect one or two additional digits accuracy.

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