       \right) dx\,dy

    If the case *kind* ='geom' is chosen by the user, a simple geometric
    approximation is used (weighted average of the triangle normal
    vectors), which could improve speed on very large grids.

    References
    ----------
    .. [1] Michel Bernadou, Kamal Hassan, "Basis functions for general
        Hsieh-Clough-Tocher triangles, complete or reduced.",
        International Journal for Numerical Methods in Engineering,
        17(5):784 - 789. 2.01.
    .. [2] C.T. Kelley, "Iterative Methods for Optimization".

    