Normalize numerator/denominator of a continuous-time transfer function.

    If values of `b` are too close to 0, they are removed. In that case, a
    BadCoefficients warning is emitted.

    Parameters
    ----------
    b: array_like
        Numerator of the transfer function. Can be a 2d array to normalize
        multiple transfer functions.
    a: array_like
        Denominator of the transfer function. At most 1d.

    Returns
    -------
    num: array
        The numerator of the normalized transfer function. At least a 1d
        array. A 2d-array if the input `num` is a 2d array.
    den: 1d-array
        The denominator of the normalized transfer function.

    Notes
    -----
    Coefficients for both the numerator and denominator should be specified in
    descending exponent order (e.g., ``s^2 + 3s + 5`` would be represented as
    ``[1, 3, 5]``).
    