FFC violates immutability of quadrature elements

[Martin Sandve Alnæs]

In ffc/analysis.py, the lines:

    for element in form_data.sub_elements:
        if element.family() == "Quadrature":
            element._quad_scheme = scheme

violate the immutability property of elements. This is not safe, and will cause problems if the same element is used in different forms with different quadrature schemes.