def centroid(positions):
    """
    Centroid is the mean position of all the points in all of the coordinate
    directions, from a vectorset X.

    https://en.wikipedia.org/wiki/Centroid

    C = sum(X)/len(X)

    Parameters
    ----------
    X : array
        (N,D) matrix, where N is points and D is dimension.

    Returns
    -------
    C : float
        centroid
    """
    return positions.mean(axis=0)
