Calculate the volume of a tetrahedron, given the four vertices of vt1,
    vt2, vt3 and vt4.
    Args:
        vt1 (array-like): coordinates of vertex 1.
        vt2 (array-like): coordinates of vertex 2.
        vt3 (array-like): coordinates of vertex 3.
        vt4 (array-like): coordinates of vertex 4.
    Returns:
        (float): volume of the tetrahedron.

def vol_tetra(vt1, vt2, vt3, vt4):
    """
    Calculate the volume of a tetrahedron, given the four vertices of vt1,
    vt2, vt3 and vt4.
    Args:
        vt1 (array-like): coordinates of vertex 1.
        vt2 (array-like): coordinates of vertex 2.
        vt3 (array-like): coordinates of vertex 3.
        vt4 (array-like): coordinates of vertex 4.
    Returns:
        (float): volume of the tetrahedron.
    """
    vol_tetra = np.abs(np.dot((vt1 - vt4),
                              np.cross((vt2 - vt4), (vt3 - vt4)))) / 6
    return vol_tetra