Fix and small refactor for butterworth filter

Fixed a typo in line 171 of subscriber_imu.cpp

const double wa = (2.0 / dt) * tan(wc * dt / 2.0);  // Pre-warped frequency

-->:

const double wa = (2.0 / dt) / tan(wc * dt / 2.0);  // Pre-warped frequency

Refactored line 169-174 of subscriber_imu.cpp

    const double wc = 2.0 * M_PI * cutoff_frequency;    // Analog cutoff frequency (rad/s)
    const double wa = (2.0 / dt) * tan(wc * dt / 2.0);  // Pre-warped frequency

    const double k = wa * dt / 2.0;

->

    const double sampling_frequency = 1.0 / dt;
    const double frequency_ratio = cutoff_frequency / sampling_frequency;

    const double k = 1.0 / tan(M_PI * frequency_ratio);