fix __array__wrap in DiscreteField

[duarte-jfs]

While using numpy==2.0.0, I have noticed the deprecation warning in DiscreteField.array_wrap. In their documentation, they now enforce that the signature of array_wrap to be __array_wrap__(self, arr, context=None, return_scalar=False). Therefore I propose we change:

class DiscreteField(ndarray):
    ...

    def __array_wrap__(self, out_arr, context = None):
        # invalidate attributes after ufuncs
        return np.array(out_arr)

to:

class DiscreteField(ndarray):
    ...

    def __array_wrap__(self, out_arr, context = None, return_scalar = False):
        # invalidate attributes after ufuncs
        return np.array(out_arr)