matrix-vector multiplication.

    :param eps:
        Relative precision.
    :type eps: float
    :param m:
        Matrix row dimension.
    :type m: int
    :param n:
        Matrix column dimension.
    :type n: int
    :param matvect:
        Function to apply the matrix transpose to a vector, with call signature
        `y = matvect(x)`, where `x` and `y` are the input and output vectors,
        respectively.
    :type matvect: function
    :param matvec:
        Function to apply the matrix to a vector, with call signature
        `y = matvec(x)`, where `x` and `y` are the input and output vectors,
        respectively.
    :type matvec: function

    :return:
        Left singular vectors.
    :rtype: :class:`numpy.ndarray`
    :return:
        Right singular vectors.
    :rtype: :class:`numpy.ndarray`
    :return:
        Singular values.
    :rtype: :class:`numpy.ndarray`
    r0