“The only obligations imposed on the member states were those contained in the directives. The interpretation of those directives, and the way in which they are implemented by the member states, must take into account the fundamental principles of the VAT system, including the principle of fiscal neutrality, both in the sense of equal treatment and that of neutral tax burden. But those principles are given effect only by the directives, as so interpreted and applied, and it is therefore according to the provisions of the directives that the basis of assessment falls to be determined.”