Return the average vector of a list of vectors.

def ave_list_v3(vec_list):
    """Return the average vector of a list of vectors."""

    vec = Vec3(0, 0, 0)
    for v in vec_list:
        vec += v
    num_vecs = float(len(vec_list))
    vec = Vec3(vec.x / num_vecs, vec.y / num_vecs, vec.z / num_vecs)
    return vec