shear_force method BUG

[howie12345]

        max_force = max(
            map(
                lambda el: np.max(np.abs(el.shear_force or 0.0)),
                self.system.element_map.values(),
            )
        )

in the shear_force function, el.shear_force is a numpy array, el.shear_force or 0.0 will return ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()